How many users can Craft handle?
It depends more on how you are going to create and display them. I had to import 20k elements this weekend and I had to be careful to store them step by step. 

Depending on your server/hardware - you can't just insert 10k elements in one request. There is a function App::maxPowerCaptain() to disable the timeout but I still faced overflow errors. 

And of course you can't display so many users in one single page without using a pagination or Ajax

Besides that - Craft is able to handle such an amount of elements if you can do it in your code as well