Separating customer data into different databases but serving the same business logic.

How would we go about developing a backend for a project management SaaS/ERP product in Xano (let's take Asana as an example) that has to serve many different customers and their employees, where everyone is using the same version of the product (business logic, schema, APIs) but where the customer data is separated into different databases?
The customers would have an unequal number of users, so they would require a varying amount of resources, depending on the usage. How would we distinguish between those that have 100 users from those that have 10 and make sure everyone gets good performance and is billed properly for the consumption of the resources.
How would we go about architecting this in Xano and on what plan would it be possible?
Thanks.