Most articles that you about how to plant specific plants do not fail to mention as to what should be the spacing between plants. The spacing between plants is important as it helps in deciding the overall growth of the plants; too little space and they end up competing for the same nutrients, for soil, and even for sunlight. In such cases, the growth can be severely limited, and you may be disappointed in the end with the plants that finally emerge.
Now, combine this with the advantages of growing multiple plants together (if done with good planning and knowledge, you can reduce the competition between plants, with some plants maybe depending more on the nutrients from soil and others less, and in excellent cases, some plants replenishing nutrients in soil that other plants have extracted). The question that emerges is what should be the spacing between plants when you have different plants.
This site (link) attempts to explain this, and even provide an online calculator to explain in more detail:

Mixed planting, or companion planting, can offer benefits over monocultures:
o Mixed crops often have higher yields than monocultures because different species use different resources, making more efficient use of land.1
o Mixed plantings often have fewer pest problems than monocultures because pests have a harder time finding suitable hosts, or because diverse plantings provide better habitat for natural enemies.
Although mixed plantings are common, practical resources for those who grow mixed crops are few; production guides and extension materials are often based on the assumption of monoculture.

So, it is actually a recommendation that if you have a kitchen garden where you grow plants and fruits for your own use, then consider the option of mixed planting, and if you that, do refer to the linked article.

Got this information from another group via email, so don’t know what the source of this information is:

Organic Landscaping in Planting Beds
“Weed control without chemicals” may conjure up unpleasant thoughts of getting down on your hands and knees in the yard on a hot day and pulling out stubborn weeds by hand. But weed control without chemicals needn’t be so primitive, so mindless. In the resources to which I provide links below, I discuss a variety of weed-control components to be used in planting beds. They should be used in conjunction with each other. Weed control without chemicals can be tough work, and I want you to work smarter, not harder. No need to revert to the Stone Age!

Weed Control Without Chemicals: Laying the Foundation
The first smart idea in a project of weed control without chemicals is to prepare the plot of ground in question. Just as in a construction project, laying a good foundation is of the utmost importance. In this context, by foundation” I mean the state of the ground where your plants will be growing. Implement these ideas before you plant, so that you’ll get off to a smooth start in your project of weed control without chemicals.
1. Killing Weeds Through Soil Solarization
Soil solarization is a preventive, organic method of killing weeds — before they even sprout! The advice below is meant for homeowners wishing to start out with a clean slate, re-landscaping a weed-filled patch of land in such a way as to reduce to a minimum the hassle of weed control in the future.
2. How to Lay Landscape Fabrics
For those in need of soil solarization (see above), installing landscape fabrics can be considered Step #2 in the project of weed control without chemicals. For those with less weedy properties, it’s Step #1. I promised you above that weed control without chemicals wouldn’t mean going back to the Stone Age, and landscape fabrics are a case in point. Landscape fabrics are a hi-tech ally in the battle against weeds.
3. Using Garden Mulch
As the final element in a good “foundation” for your bed of annuals, perennials or shrubs, you should apply garden mulch on top of the landscape fabric. Many skip right to mulching in the project of weed control without chemicals, but I do advise you to lay the landscape fabric first: it will lengthen the life of your garden mulch.

Q&A on Mulching Gardens
Weed Control Without Chemicals: Pre-Emergent and Post-Emergent Organic Herbicides
“Pre-emergent herbicides” is a fancy way of referring to herbicides that attack annual weedy plants at the source: their seeds. That is, they act to inhibit seed germination before the plants in question even have a chance to announce themselves to the world above! Corn gluten is a pre-emergent herbicide used for weed control without chemicals. Remember, corn gluten will inhibit the seed of “good” plants from germinating, too, so don’t use it in planting beds where you’re starting plants from seed.
By contrast, you apply “post-emergent herbicides” only after the antagonists have appeared on stage. For weed control without chemicals, try vinegar as a weed killer. Even plain old household vinegar is effective if you have only young weeds to deal with in your planting bed. It’s the acetic acid in vinegar that gives it its herbicidal abilities. The higher the percentage of acetic acid in the vinegar, the better. Vinegar used for culinary purposes is relatively low (5%) in acetic acid, but you can boil it down to increase its strength — a provision that is necessary
if you’ll be applying it on full-grown weeds. Alternatively, you can try to buy the super-strength vinegar in stores that cater to the agricultural community.
A word of warning, though, when using vinegar as an herbicide: apply it directly onto the weeds, since vinegar is a non-selective herbicide and will harm plants that are inadvertently exposed to it. For this reason, vinegar isn’t especially effective for battling lawn weeds: you’d almost inevitably end up with too much collateral damage to your grass. But vinegar is a fine organic alternative in planting beds, as long as you watch where you’re spraying!
Weed Control Without Chemicals: In Case You Still End Up Pulling Weeds
With landscape fabric and garden mulch in place, the bad news is that, even then, you may still get weeds. But the good news is that those weeds will be very easy to pull out. Pulling weeds embedded in mulch is not nearly as difficult as pulling weeds embedded in soil. Thus with a good “foundation,” you may not even feel the need to bother with the corn gluten and vinegar: 5 minutes of easy pulling here and there should get the job done.
One exception to this may arise: if the integrity of the landscape fabric has been compromised, weeds may strike down roots in the soil beneath, making them difficult to pull out. In this case, water the area in question beforehand. It’s usually easier to extricate weeds from moist soil than from dry soil.

CELERY
Celery is often referred to as a vegetable, but it is actually a herb. It is a biennial plant, growing well in moist soil in a sunny position.

Growing conditions:
Thin your plants out by transplanting them, and water well every 3 days and pile compost and sand around the plants base. The seeds can be collected as soon as it is ripe and kept in airtight bottles until ready to plant.

For drying, plant should be pulled up that are going into the flowering stage, or pick off the outside leaves throughout the year

Celery and leeks enjoy growing next to each other, as do tomatoes and bush beans

DOMESTIC USES
An excellent compost maker. It helps to break down the compost and can be layered
between lawn clippings to break them down quickly.

COSMETIC USES
An infusion of celery leaves makes an excellent cleanser for oily skin. Pour 2 cups
of boiling water over a cup of celery leaves and allow to cool, then strain. This can be added to the rinsing water for oily hair

MEDICINAL USES
Celery is great for weight loss and rheumatism. It also relieves flatulence. An infusion made with celery leaves or seeds is an effective diuretic

CULINARY USES
Celery, dried or fresh, can be used in soups, salads, stews or bouquet garnis.
The flavour combines well with most vegetables.
Celery seeds can be used to flavour savoury dishes and also in salad dressings, as
it cuts down on salt intake

The previous article was about growing tomatoes in containers. But suppose you have the space to grow tomatoes in soil, and want to figure out what all you need to do to get a healthy output. There is plenty of information that would help you in this process; after all, you need to know about which types of tomatoes, what are the pests and blights that could affect your plants, what is the best type of soil, what is the amount of watering and sunlight that should be given to the plants, and so on. Further, if you get tips and points that you should avoid doing, those will help you a lot. Here is the link to an article that should help you in this regard:

# Good first-time growers’ varieties include Better Boy, Creole, Big Boy, Early Girl, Brandywine, Celebrity, Lemon Boy, or just about any cherry or grape tomato variety.
# Plant several varieties rather than all of one type– this ensures a steady harvest. As a rule of thumb, it’s good to have two plants for each member of the family who will eat tomatoes. If you plan on canning or making salsa, use up to four plants per person.
# Prepare the garden bed by adding lots of compost (5 to 8 pounds per square foot/25 to 40 kilograms per square meter) to the soil. Turn compost into the top 3 inches (6 to 8 cm). Tomatoes demand a growing medium rich in organic matter. If you don’t make your own compost, use store-bought compost or composted manure available in the 40-pound bags. Compost or Manure is usually less than US$5 per 40-pound bag.

The referred article provides help from the process of selecting the tomato plant / seed, and then goes onto providing an estimate of how many plants you should have. You learn about the tomato spacing, watering, providing fertilizer, support for the growing plant, when to pick the tomato, warnings and things to avoid, etc.