Hi Federico,
Usually, Free app can have as many users as you want, but there are some downsides of course: You cannot use schedule events for example and sleep mode that you already mentioned. Also, free apps has sometimes problems that cannot be waked up from sleepe mode, so you need to contact support to resolve the issue. Also, not sure how free app behaves when multiple users use the app at the same time, can it handle, for example, 100 users.
For the 2nd part of the question. Yes, if you pay for 10 users lets say, only 10 users at the same time can use the app.
I hope this helps