How many devices the Volumio plan included in Integro can handle?

The Volumio plan built-in fits for the Integro device itself.

To handle multiple devices in addition to Integro, 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.