It is the user that create/read/update/delete the tenant, not the other way around, so he can have multiple tenants. And in theory, a user can exist without any tenant if you desire so. You just need to implement a role based authorization system in your app.
App wide super-admin can be implemented as a kind of super user that has access to all tenants, but it's better to have it's own class so you can also assign different roles to it.
It is the user that create/read/update/delete the tenant, not the other way around, so he can have multiple tenants. And in theory, a user can exist without any tenant if you desire so. You just need to implement a role based authorization system in your app.
App wide super-admin can be implemented as a kind of super user that has access to all tenants, but it's better to have it's own class so you can also assign different roles to it.