A full grown T. natans grows to over 18". One or two would do fine in a 55 gallon tank, but if keeping more, you'll need more hiding logs, so you'll need more room. They like to spend time together hiding in enclosed spaces, so artificial logs and tunnels are a must, IMO. I'm considering building PVC pipe tunnels along the inside of the tank, to see if the eels would like them.

Katrina