The bounce of the cannonball is treated exactly like a special kind of template, which we assume to be about the width of the cannonball itself.
On most occasions, any model whose base is between the point where the cannonball first strikes the ground and where it eventually comes to land is hit automatically! However:
A maximum of one model per rank struck can be hit. If the cannon is in the target's flank, the target's files count as ranks.
If the cannonball bounces into a monstrous infantry/beast/cavalry or monster, that model suffers a hit. However, if the monster or monstrous infantry/beast/cavalry model is not slain, the sheer bulk of the creature robs the cannonball of all momentum and the shot travels no further.