I had well over 50 in a 10g tall at one point. As much as I loved them, I have never had any pet that required so much attention.
You will have to constantly and successfully hatch tons of baby brine shrimp (I was using a 5g that was literally pink with them) - they ONLY eat live food. They need to be fed multiple times per day. Consider what you will do if you need to go away.
They aren't cheap: you can order them from Ken at Sea U Marine - he gets them from seahorse corral in the US - they will be app. $35 each.
Alternatively, you can organize your own cites paperwork and pick them up at the airport. They will about $10 cheaper, and you need to order min. 100 specimens.
They are a very social species, imho you need multiples in a small tank. There areny't many things you can have with them - I had pederson shrimp and a pair of pearly jawfish and some macroalgae and photosynthetic gorgonians.
You need to be diligent with water changes and monitoring water parameters, in part because of the high bioload, in part because its a lot of work to maintain stable parameters in a small tank.
You need to be able to provide some flow, without the possibility of H. zosterae being pulled towards pump, etc. I had to modify almost all of my equipment used on this tank - I think I was running two heavily modified ac70s, and at least two air stones. You need to make sure the tank temp will never exceed mid 70s, or you will lose many if not all seahorses.
Do lots of research. And I mean lots. I would have the tank set up for at least a few months before putting them in. As much as I loved this tank, i will never do H. zosterae again. It's like having a part time job, and even if it's enjoyable there is never a day off.