As described on the pricing page, it is not based on the number of users. The principle is that if your app is a prototype, it can be tested for free. But when it is being used for real (Deployed), you should be on a paid plan.
As a practical constraint, a prototype app can have no more than 10 users. However, a deployed app with 5 users should still be on a paid plan. Hope this clarifies.
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