Glock, and a few other have aftermarket gimmick drum magazines that hold 50 or more rounds. If you're talking about a 9 MM AR platform gun, C-Mag makes a 100 round drum that fits. The problem with all of these type of magazines is they are heavy and unwieldy. Some also require lubricated ammunition.
You're best bet for high capacity in a handgun would be Glock, with their factory 33 round stick magazine. They're relatively cheap and available, work well, and don't make your pistol feel like you hung a boat anchor from the magwell. They will also work in all of their double stack 9 MM models. Glock 26, 19, or 17.