You get 18 cannon balls for the cannon gun. (Cannon guns can be found in crates that fall from the sky with green smoke.)
(If you own the old gamepass -> https://www.roblox.com/game-pass/7135614/More-cannon-balls you dont have to buy the new one)
Charts
Roblox disabled public gamepass sales visibility in late July 2020. Sales data collected up to then is still available here for reference.