Belch can be used once, under normal circumstances.
The user lets out a damaging belch on the target. The user must eat a Berry to use this move. ~ Serebii
As the attack's description clearly states, a Berry is needed for one use of Belch. The only way to use Belch successively is with either Recycle or the Harvest Ability. However, only Trevenant, Exeggutor and Tropius get harvest, and none of them can learn Belch. According to the DB's moveset searcher, the only Pokémon that can learn both Belch and Recycle are Trubbish and Garbador.