From everything that I have heard, dwarf seahorses will breed quite often. There is one poster who wrote into Bob Fenner who said that he cannot keep the population under control. He said he was up to 51 sea horses in his 2 gallon aquarium. Yikes.
Other sources that I have read say that they do best in smaller tanks. This may sound wrong at first, but the explanation was quite sound. Because the seahorses eat so slowly, it takes them a while to get to the food. In a larger tank, they will be hunting for their food forever. To counteract this in a larger tank, you will have to put in a lot of food. The end result of this is that you will spend all day removing the uneaten food or you will get a lot of decay from the food you were not able to find and remove. Obviously, this is not good, as the water quality will be bad.
A full sized adult only gets 1.5 to 2 inches. This is not very large at all. I would think you could keep 3 pairs of dwarf seahorses in a 3 gallon comfortably.
Now the unfortunate part. Dwarf seahorses demand quite a bit of care. Their feeding can be a bit rigorous. Most people are only able to get them to eat live brine. The dwarf horses must be fed daily, preferably, twice to 3 times. This means that you must set up a brine shrimp hatchery and be very up on it. To me, it sounds like you need to be able to devote a lot of time to a set up like this. I don't know about you, but I have nowhere near that amount of time. Unless you work from home, it would seem to me that their needs would be a bit hard to meet. Unlike other horses, where they are larger and easier to feed, and whom you could get trained on frozen, these guys seem like a lot of work. You need to seriously ask yourself whether you have that kind of time every single day.