Carcinus will often aggregate together so keeping them in groups is not a problem, if you provide optional shelter for them then this can limit any altercations that may occur.
When starving the animals it should be noted that any crab that molts will be targeted by the remaining animals and so can compromise the entire groups starvation process. When i have needed to starve large numbers of crabs at once I have put them in individual pots within a large tank, so that the surrounding system water is the same but the animals remain separate. This is not ideal in terms of holding coditions but it prevents any predation when only a small amount of space is available.