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.