The strategy guide says it can be between 18 and 22 tonberries before tonberry King shaws up. I havn't gotten to the Centra Ruins yet, so I havn't had a chance to try it.

But remember, Tonberries have a lot of HP and are really powerful. Take advantage of their slow speed and pummel them to death with Diablos and other strong GFs.

Also, a thought occurs. Why not cast Aura on Quistis, and then use Degenerator on them continiously? They are regular monsters, after all, so they can be killed instantly. Although it will take a fair amount of Aura spells.