You don't need more than one, and it's usually easier to have a single registry with a single site so that you aren't spamming your guests with different website links.
However, sometimes there's an item you want on one website that isn't available on another. Kristen Gosselin, expert celebrity wedding and events planner, says that thankfully 'you can link other registries so that all of the requested gifts show on one platform' with registry services like Zola and The Knot. She adds that you can 'essentially price shop for your guests', so if something is cheaper on one site you can save your guests some money.