Seahorses are protected under the CITES act and as such any sold in the shops should be captive bred. Captive bred seahorses are easy to keep because they readily eat frozen foods and don't need live foods. This means you can feed them on frozen brineshrimp, mysis, and other sorts of marine foods. They have a high metabolism and need to be fed several times a day.
A pr will be fine in a 100litre tank that has a reasonable filter. As mentioned by Anguilla they don't tolerate lots of water movement so you need good filtration with low water movement. External canister filters or a trickle filter are the best choices. Internal power filters are generally too powerful.
I wouldn't buy any from an online store and would go to a proper shop to buy them from. When the shop assistant catches them make sure the seahorses are not lifted out of the water. They should be scooped out in a container or simply chased into a plastic fish bag that has water in it.
General tank maintenance is the same for seahorses as it is with other fishes. Regular partial water changes and good filter maintenance. Make sure the tank has an established filter before you get them. Keep the nitrates as low as possible and don’t keep any fast moving fishes in the tank with them. Small gobies and bigger cleaner shrimp can be kept with them.