In every game that it has appeared in so far, Cannonballs appear as a small pile of four cannonballs. They serve as ammo for all variants of the Cannon.
The amount of ammo in each box of Cannonballs depends on the game and (in some cases) the difficulty level. In Serious Sam 1, HD and Next Encounter, each pile gives the player 4 Cannonballs (8 in Tourist, Easy or Serious Difficulty). Serious Sam 2 gives the player 8 Cannonballs per pickup and Serious Sam Advance gives the player 5 Cannonballs per pickup.