At this time we have a small number of paying customers (each of which has one or two apps, often one for their clients and one for them to administer with).
For each customer we allow them a maximum number of clients (which we restrict in the app). We then invoice them once per month to cover our licences (Glide, Integromat, Clicksend and other services that we may require for their solution) and our ongoing support and app maintenance. They can add more clients in chunks of 20 or 50 and this increases their monthly invoice amount.
We will be able to operate like this up to about 20 customers i.e. 20 invoices per month but beyond that we will need something more automated. We are not charging individual subscriptions for any current or planned apps.
Hope this makes sense?