Technically you only need to register in one instance and can interact across lemmy and kbin instances, but you can register the same username in various instances in case of defederation. Lemm.ee federates with all and all instances federate with it for example, beehaw.org doesn't allow signups and has defederated lemmy.world and sh.itjust.works.
You can have a display name different from your username (just learning all this myself). Then you can use the same account across instances with different display names or vice-versa.
Love the usenet-ish federation. Now if only we had federated killfiles linked to our accounts. It'd be cool to raise/lower scores privately to bubble things up you care about and kill what you don't.