I do load testing on a regular basis between each major release to make sure I'm not losing performance over time, though my testing is not exhaustive of all features and all scenarios.

I've also done load testing comparisons of mojoPortal vs other .NET apps and mojoPortal compares favorably in my testing.

As far as specific numbers of user and requests of course that depends on hardware. Capacity planning requires a lot of effort. There is a very good article "How To Perform Capacity Planning" which outlines the steps involved with coming up with hard number predictions. As you can see its a lot of work to really do it right for all features. I have not found time to do all of that.

What I generally do is periodically dedicate some time to performance optimising, which I do using Red Gate Ants Profiler in conjunction with load testing. Red Gate helps find code that is slow and should be optimised and load testing verifies if the change had any impact.