There is no universal Lemmy rule or anything against that, but any instance could hypothetically set their own rule stating “You are not allowed to have an active account on any other instance” or something. With that said, I have never seen an instance with a rule anywhere near that (granted, I have only read the rules for a handful of instances, not even close to most instances). It is unlikely you will ever see a rule like that, but it could technically happen. Make sure to thoroughly read the rules of whatever instance you are considering signing up for.