this depends on the configuration and hardware you use, as well as the modules installed. Large sites use server farms and separate datebase servers (or server clusters).

Large sites like dotnetnuke.com can handle thousands of users simultaneously, but it depends, how much caching can be used.

This question comes up so often, and has so many variables.  I know there have been many falmes about adding any sort of "Call Home" functionality to DNN....but how about a voluntary submission of non-identifying statistics--kind of like some companies "An error has occured, please click here to submit details to the vendor so we can improve our product?"

What if the Core Team added a "statistics submission facility?"  Each installation already logs enough data to estimate load from.  Pare the logs down to non-identifiying info--maybe even just statistical summary...  Then add to that simple header like DNN version, hardware (CPU count and type, RAM, etc...), whether IIS and DB are on the same box, any discernment to webgarden, etc), total number of portals, total number of users, total number of modules...

There are a LOT of DNN installations out there!  If only 1% volunteered to click the "Submit Statistics" button, then we'd have a LOT of observations!  We could give some interpolated charts to guide planning & expectations!  Isn't that better than vague guidance and "guesstimates?"