That's entirely up to you, it can be the same username if you want. Speaking as an instance admin, there is no problem with users creating multiple accounts across instances, even if they're the same username.

Spam would be creating as many usernames as you can on any given instance (e.g. trying to register 100 users on lemmy.world because reasons) - there's obviously a problem with that. Creating you@instance1, you@instance2 and you@instanceN is perfectly fine.