As many as you want. 5, 50, 500 ...
Type the number and press the button.
Currently we have reports about creating a few thousands of users in one run.
Actually the limiting factor are the resources of your server. The most resource intensive part of the procedure is the creation of the avatar and cover images.
CAUTION : Since the server is a limiting factor, don't test the component for the first time with a big number. It could be too big for your server and overloaded it. Start with small numbers and see how it goes.