Edit because the previous answer was wrong. It may be 7 years late but eh.
Belch may be used more than once per battle. For it to be used, a requirement needs to be met. That requirement is that there is a berry that was already eaten. The requirement is not that there needs to be an existing berry for the move to consume the turn its used. So as long as a berry was already eaten, a Pokemon may use Belch as long as it is in battle.