Well, for your sized tank (Nexus13) I would only put two in. They have no known predators, so compatibility isn't a problem. Any more than that and like what Drock169 said-they will starve.
As far as breeding goes, I have had them mate...they laid a string of about 500 eggs (at least that's what it looked like-grain of sand size or smaller). It's hard to tell if they hatched or not because they are so dang small. I put them in a 10 gal tank that I keep just for "farming" aptasia for them to eat. (Strange-I know). I thought I would be able to dig them out when someone wanted some, but I have played heck trying to find the buggers. I know they are in there-aptasia disappears all the time-but I NEVER see them.
Although they have both male and female organs, they need at least two to breed. With that, they are "pack hunters" (have you seen a Berghia? There's nothing "pack" about them-but oh well...). You need to have more than one so that while one is distracting the aptasia, the other can swoop in and bite it. It's also REALLY important how the Berghia are placed into the tank (that was another lesson learned the hard way...
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I have been working directly with the breeder to start bringing these guys into the Western half of Canada, so if we can get enough people interested, I can place a big group order for everyone. (It helps on the price). If Kevin is ok with spearheading the order, everyone can contact him to let him know how many Berghia they want and pay him for them (the final price will depend on how many we are ordering). We'll set a predetermined cut-off time and I will place the big order. (Usually, if we can get the order placed by Monday we can have the shipment in less than a week.)
The only thing we need to have an understanding of is that when I get the package, they need to be picked up the same day. They come in little containers that will keep for a while, but I don't like pushing it.
Hopefully, I have answered all of the questions, but if not, please feel free to post or PM me.