Devise does not impose anything on the number of tenants. I run a Rails app with multi-tenancy and SSO support with Devise. As someone has mentioned, although it does a lot of the work and it keeps well organized, you still need to do the UI for configuring and some backend logic.

Devise doesn't support SSO, but it supports omniauth, which supports SSO, right?

You certainly can write and maintain code to add multi-tenant sso, but since omniauth does not support any configuration storage, you have to add that yourself.