A twenty is too small for standard seahorses but a WAY too big for dwarfs.
You can put 3 or 4 dwarfs on a sliver dollar.
With dwarfs, the biggest problem is the fact they don't normally hunt for their food, but rather sit perched waiting for it to come close enough to them to snick it up.
For a large tank that means a WAY too many enriched brine shrimp nauplii have to be placed in the tank to ensure a density that gives them sufficient food in a feeding.
Then, before the next feeding you need to remove the uneaten food because the nutrient levels decrease too fast to leave them in for the next feeding.
You remove, then replace with newly enriched nauplii.