After the Civil War event in the Marvel Comics universe, registration is required for super-powered beings.
Is there a cap on this? Do some people get out of this by being 'useless'? Are people enlisted, as well as registered?
Registration is no longer required! Huzzah!
However, back during the heyday of registration, Avengers: The Initiative gives us a good look into what powersets are registered, and what ends up happening.
Cloud 9 is the example that immediately comes to mind. At the beginning, she has no interest in superheroics, but the Initiative program turns her into a sniper. Butterball, on the other hand, washes out of the program since he's not very useful in combat.
To answer your questions: If there is a cap, it would only be to weed out people with purely ornamental powers. Even these people might have to register, although training seems unlikely. As shown in Avengers: The Initiative, registrees are trained to become superheroes, which serves a loose form of enlistment.