I think this is one of the things we need in a primer that is prominent wherever people talk about lemmy/kbin/meta verse in general.
You only need to sign up to one instance. It helps if they have the communities you want already there but if they don't, you can still access them.
I cannot comment for lemmy but on kbin there's a search icon and you can just type community@instance. For example this community on a remote instance is [email protected]. I'm reading your comment and typing this from my own kbin instance. It's how this was designed to spread the load.
So pick one to stick to and add all the communities there.