Not all USPSA stages will need max rounds, but ours average 25 rounds/stage. Typically 5 or 6stages, so 125-150 rounds, if there are no re-shoots and all steel goes down 1st shot.
As for needing to reload ammo, that is up to you. Shooting 45acp drove me into reloading. 9mm would have taken longer, but reloading would have happened regardless.