I think 20 players would be just too unwieldy. If you're going to have that many, I think an (appropriately named) shootout tournament would be better. There could be 25 players, split up into five groups of five. To make it more interesting and realistic, the second round could have "memory" of the first round, so if everyone dies in the first round, no-one progresses to the second round, and if the winner is loaded in the first round, they keep their ammo in the next round. It'd make it sort of like 25 person one round tournament in which you are restricted in who you can shoot, making it easier.
I like Nich's idea better since it'll be hard to get 25 on the forums at once. So break it down into waves or groups on whatever would be nicer.
What do you guys think about adding something that can defend a bazooka. Like a shield but it can only be used X amount of times. So if you use it and nobody shoots at you, it's a complete waste.