Have a quick question: What is the logic behind salesforce developer edition instances license limits. One developer instance has only 2 salesforce licenses and other has 20 and another has 30?
The variant of the developer environment depends on things for which it would be used.
The general rule of thumb is the one with lesser licenses is the one where you actually code and the higher one is where you do Integration tests or UAT(Mass Login Tests)
You might wanna look at environment types in this official blog here.
Each of the different types of Developer Editions available have specific uses. Their license limits are set to facilitate those uses. The smallest, the two-user configuration, is meant for a private developer to have an org they can develop code in and have a test user they can use to test their app with. The larger ones each serve a specific purpose, such as for development, testing, demos, templates, and so on. Many of the larger types have an expiration date, but the standard Developer Edition and Partner Developer Edition do not expire as long as they're active. In addition, we can create Scratch Orgs, which also come in various sizes, again for development and testing purposes, and which always have a fairly limited shelf life.