It's really up to you - we recently decoupled the UPS from keycloak, and you can define as much users as you want. It's all possible in Keycloak itself.

We do have a demo realm, like here:
https://github.com/aerogear/aerogear-unifiedpush-server/blob/master/docker-compose/development/ups-realm/ups-realm-sample.json

And yes, that one has just one account - for simplicity reasons