Maximum numbers of Users

[Joeisonfire]

Hi!
I'm trying to make a sign-up form. Everything works smothely now, but I want to have a maximum numbers of user. For example maximum 20 users. Is it possible to automaticly remove the form from the page and replace it with some kind of text (for example: "Registration is closed") ? This, hopefully, without having to manually go into the administration and removing the form..

-Joe-