Dwarf seahorses, being much smaller than their counterparts (about 1 inch as adults), have different stocking rules. In a 10 gallon tank, you could have quite a few. About 10 dwarves in a 10 gallon would be just fine. Fusedjaw.com has a very good article that neatly outlines their requirements. A word of warning, though: because Dwarf Seahorses are so small, frozen mysis cannot be fed to them. They are about the size, as adults, of young... Erectus fry, maybe? (I don't have a factual comparison here, but more or less...) Basically, they need to eat the same sorts of food as non-dwarf seahorse fry. This means that you will need to hatch and gutload brine shrimp twice a day to feed them. On the other hand, Dwarf Seahorses are deceptively hardy, recquire much less space/money than larger seahorse species, and reproduce like bunnies. Faster, actually. 10 dwarves will quickly (maybe in a year) become 30, without any effort whatsoever on your part.
A 60g of their own would suit larger species quite nicely. If you got H. Erectus, and I would suggest either that or H. Reidi, you could technically fit 6 seahorses. Both Erectus and Reidi can be found relatively cheaply (captive bred, make sure of that) and are a little hardier than their relatives. Of course, you would need to ensure you gave them 2-3x their adult size in height, so pay attention to dimensions. A 60g cube, for example, would be a very nice size for them. Lastly, it is better to under stock, especially with seahorses, so while you could technically put 3 pairs of seahorses in a 60g cube, it would be best to just put in 4-5 seahorses and some snails and maybe a small hermit crab. If you got a larger species of seahorse, you will almost certainly need to get a chiller, too, because the optimal temperature for tropical seahorses (like Erectus and Reidi) is 72-74 F. While seahorses can technically survive temperatures up 78, Vibrio, a strain of bacteria which seahorses are particularly vulnerable to and can die from, becomes much stronger and multiplies much more quickly at temperatures above 74 F. Stronger to the point that, if any Vibrio were present at, say, 76 F, in 3 hours, maybe less, the tank would be infested with it and your SH would definitely be infected. So it is in everyone's best interest if SH are kept at temperatures below 74 at all times, which probably means you will have to buy a chiller. The websites I mentioned before have more info on seahorse care than I care to type out, and make sure to read their stuff if you do decide to get seahorses.