The question of "big" is relative. Software development is mainly about how good your computer is. If you can compile and run 100 projects in 1 second, then 100 projects in the solution are "small." So this is really a question of what works for you.
My current working solution has about 130 projects. Yes, we could break it, but we have some impressive boxes that can handle this, so the cost of having 130 projects is moderate to low and the benefits are more than the costs.
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