Many people let them keep growing in compots until they start looking crowded. But if you leave them for that long, you're going to have a much harder time seperating them out and untangling the roots when you go to repot. There's nothing special about a "compot pot" except for its shallowness. In other words, there's no special benefit in having the plants growing all in a pot together; it's just done because it's convenient and saves space. If a seedling is ready to go from its flask to a compot, it's certainly ready to go into its own pot if that's what you choose to do, so long as the pot you put in in and the media you choose will keep the roots consistently damp but not soggy.