Some options that have been used include:
* A bed of scallops
* A catch of scallops
* A haul of scallops
Ultimately, the best choice depends on the context. If you're writing about a group of scallops found in the wild, "bed" might be the most fitting. If you're talking about a group of scallops harvested by fishermen, "catch" or "haul" might be more appropriate.