There are no limits
PDM Professional can easily support several hundreds of seats, 1000-2000+ users and 20+ replicated sites (these are not limits just install sizes that I know exist). How big of a limit do you really need? If you're a monstrous global company, you really shouldn't be asking your questions here. Contact your VAR and get the hard details that pertain to your needs and your environment.
I wonder why do you want to limit the number of users?
Is there a special requirement?
From administration, you can refuse a user to log in from the settings.