Normal recommendation for standard sized seahorses like reidi or erectus would be 29g for the first pair and 15g for each additional pair.
It can be done in somewhat smaller tanks but makes it more important to keep husbandry levels up and many fail to do so as it's human nature for many of us to slack off now and then.
For dwarfs, I put 20 in my 5g and could use more.
Check the links at the bottom of "
My Thoughts on Seahorse Keeping".
While occasionally people have luck putting other fish in with seahorses, you need additional size plus a lot of luck to do so as the pathogens introduced by other fish may kill your seahorses.