The Volumio plan built-in fits for the Rivo device itself.
To handle multiple devices in addition to Rivo, keeping them inside an account (I.e. for Multiroom purposes) you need the PREMIUM PLAN
Premium plan allows up to 6 devices, with Pro features.
Anyway you can still add, on your local network, other Volumio devices with FREE PLAN and see them all, together, in the User Interface.