Unless you get dwarf seahorses, the minimum requirement for tank size is 29 gallons for the first pair and another 15 gallons for each additional pair. Seahorses prefer taller tanks, so it is best if you can get a tank 3x their adult height. Generally, this means 18-24". Personally, I like the dimensions of a 60g cube best (24" cubed), and this would allow you to have as many as 6 seahorses, if you wanted. Seahorses are social creatures; they do fine in lone pairs, but a couple articles I have read mentioned that they seem happiest in groups of 4-6. That is anecdotal, but worth considering, if you have the space/money for a larger tank.