I think I signed up for my personal domain when it was 50, and my business domain when it was 10, I'd kinda like to check. Glad I go the business mail up and running in September now!
The only way I think you can check that is
Login to your cpanel ( admin ) > Setup > Users and Group > Yes, I have users - Next > It would tell you when you need an upgrade for me it says
To add more than 10 users you'll need a Google Apps for Business or Education account,..