Single vs. Multiple Unit Test Projects per Solution?

In addition to the other (good) answers, consider that on larger project teams individual team members may create their own solutions to include only the subset of projects they are working on.

Assuming a monolithic solution, with one test project covering everything in that solution, breaks down in that scenario.

There is no definite answer because it all depends on what you work on as well as personal taste. However, you definitely want to arrange things in a way so you can work effectively.

For me this means, I want to find things quickly, I want to see what tests what, I want to run smaller things to have better control in case I want to profile or do other things on the tests as well. This is usually good when you're debugging failing tests. I don't want to spend extra time figuring anything out, it should speak for itself how things are mapped and what belongs to what.

Another very important thing for me is that I want to isolate as much as possible and have clear boundaries. You want to provide an easy way to refactor/move out parts of your big project into an independent project.

Personally, I always arrange my tests around how my software is structured which means a one-to-one mapping between class and its tests, library and test executable. This gives you a nice test structure which mirrors your software structure, which in turn provides clarity for finding things. In addition, it provides a natural split in case something is moved out independently.

This is my personal choice after trying various ways to do things.

In my opinion, grouping things when there are too many is not necessarily a good thing. It can be, but I believe in the context of this discussion it is the wrong argument for a single test project. Too many test projects with many files inside means just one with a lot of test files. I believe the real problem is that the solution you're working on is getting big. Maybe there are other things you can do to avoid having "one world"? :)