Is there a user limit on wpforo some forums do have that.
How great can the community be by counting the users?
I'm sorry, but there is no any option for this purpose. However, this can be reached, by disabling registration, after the member's count will be enough for your forum.
E.g. you'd like to have only 45 members, just allow the registration until this count will be achieved.Then navigate to Dashboard > Forums > Settings > Features Tab disable the User Registration.
A while later you can enable registration if it necessary.