For reference, we currently use Stripe for billing, Firebase for authentication, Node.js on the backend, and React on the front end.
In my experience, none of the SaaS providers mentioned help you with this, probably because it's very dependent on your product and business logic. Even with Kill Bill, which has an entitlement system[2], some amount of coding would be required to manage and enforce states like "max amount of users reached for that plan" (the logic would be abstracted from the billing though).
- [1] http://killbill.io/blog/subscriptions-entitlement-billing-an... - [2] http://killbill.io/blog/blockingstate-abstractions/