How should we organize VSTS repositories?
From the perspective of build automation, I find that it is better to have multiple repositories.  This allows for smaller configurations and more granular control of your build/release process.  You can allow references and pull in source code to build or release (of course you can also ignore source code as well in VSTS build pipeline).

This leads to being able to release just the project you want or need to your desired environment.  You can, of course, pull in artifacts from other builds and include them in your release if you need to.

To summarize, I tend to go as small as possible and only group things into the same repository if they are indeed part of the same project and no part of the project can be used elsewhere.