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.