Suscription with limit, posible?

Hi, i’m running an event’s website in wordpress. I’m actually using the following tools:
I´ve got 150 people registered (as “suscriptors”) using the plugin Register Plus
Each user manages an extended profile using the plugin Simy User Extra Fields

What I need now is to let my “suscriptors” choose an activity from a list, but each option can hold a limited amount of suscriptors (“Activity A” has a limit of 30, so when that amount is reached, the option should automatically block or dissapear). Of course I must have further access to the lists per activity.

Is there any plugin for that or similar? I prefer having the option available in the profile, or on the desktop, but if that’s not posible it could work having a widget available on a Page or in the Sidebar.

Thanks to anyone that can help!!