Translation:The Swedish Academy has eighteen members.
Yes. You can be ledamot of things like the parliament or a 'board' of some kind – here you can be either medlem or ledamot, where ledamot is more formal. (we normally use ledamot with the Swedish academy and any parliament, but for boards, both are used more interchangeably).
But for bands and informal groups, you can only be medlem, not ledamot.