5 seahorses in a 12g? That seems like a lot IMO. I woud definitely stick with the 2 for a while and see how it goes. I absolutely adore my seahorses, but they are definitely not easy and I have my 24g sharing water w/my 125. Not sure what kind of ponies you have but the adult size can be over 12in for some some (including the Kudas I have.) Keep in mind the adult size of yours when deciding how many will fit in your tank.
JMO
Good luck and post some pics for us please.
