Yes, I agree that they are very social. I will not say that a single seahorse can not be kept because I had a mated pair for 2 years. Then the male got gas bubble disease and died. The female did fine without him. It was several months before I bought more seahorses.
I personally would never purchase just 1 because of their natural social personalities. They really do enjoy one another.
Another consideration is that you the owner will miss out on multiple personalities. They are fun to watch their interactions.