The Shrimpy Developer APIs are designed to support an unlimited number of users. Each user is intended to be a 1-to-1 map to a real person who is using the service you are developing. Therefore, if you have 5,000 users in your application or service, you should have 5,000 users registered with the Shrimpy Developer APIs.
Shrimpy will charge credits for each user that is currently being managed by the developer. The number of credits charged is based on the pricing outlined here.
Once the "Remove User" endpoint has been called for a user, the user will be permanently removed. That means the developer will no longer be able to access that user, execute trades for that user, or access the user's exchange accounts. After a user is removed, they will no longer cost credits. That means you will only pay for users that have been created, but not removed.
Note: Shrimpy only charges for users, not accounts. Each user can link multiple exchange accounts (at the time of writing, this is 20). Linking multiple exchange accounts does not cost extra. Even if they execute trades across multiple exchanges, it still counts as 1 user.