As Bart has already mentioned you can set up multi tenancy using the entity access in the data model. In order to control the license pools and number of users per tenant use the License pool manager app: https://appstore.mendix.com/link/app/License%20Pool%20Manager
What i did is have a tenant entity which inherited the license pool entity and associated all users to a certain tenant pool. This gives you the ability to control the number of users per tenant and the number of concurrent users.
If you have any other questions then let me know as i am currently working on a system very similar to yours.
With instance access on each entity you can arrange a multi-tenant solution within 1 Mendix instance. You would need an association to a 'Tenant' entity from each entity (direct or indirect) and also for user accounts to indicate to which 'tenant' they belong.