As for numbers, it depends on how bad the aiptasia is, how many potential berghia predators (wrasse, shrimp, etc.) you have, your bedget, how quick you want the aiptasia gone, etc.
That said, Ive found Reeftowns recommendations to be pretty accurate.
1. At least 1 Berghia per 10 gallons or 2 Berghia per 10 Aiptasia plus one or more per additional 10 Aiptasia for a small infestation, but NEVER less than 2 pair of Berghia. Berghia NEED each other to Hunt, Feed & Breed. If you're not sure, it's best to overestimate rather than underestimate.
2. It is important to realize that Berghia are COLONIAL HUNTERS, COLONIAL FEEDERS & COLONIAL BREEDERS. Therefore, it is important to keep them together and assure there are enough Berghia to be able to find one another for breeding. It is necessary to have several Berghia attack a large Aiptasia at the same time in order to successfully eat it. A single Berghia may spend hours circling and attacking a large Aiptasia and unsuccessfully gain access to it, when several Berghia will circle and attack it together until it's eaten. Further, the Berghia you are adding are the breeders. It's their successive generations that will be your success. That is why we recommend the Medium Berghia, as at that size and age they are laying eggs every 1-3 days.
3. For several groups of more than 10 Aiptasia, a large colony of 8-10 Berghia are needed as Aiptasia multiply quickly. It may take several generations to build a large enough Berghia colony to completely eradicate an Aiptasia infestation.
4. The more Berghia you start with the faster they find each other to lay eggs and the larger the colony becomes. Thus the faster the Aiptasia infestation declines and your aquarium becomes Aiptasia free.
