There is no restrictions in the software on a number of user accounts you can create. Therefore, in this sense, in theory, you can have unlimited number of users.
However, to keep prices low, the servers have specific capacity in terms of disk space, memory, CPU power and network bandwidth.
Hence, the recommended number of users for a specific setup is what the system can comfortably handle without performance impact or system overload.

That's said, our recommendations are for a typical usage scenarios. Therefore, your system may handle more active users or less active users, depending on your use-case.
The system monitoring shows you in real-time how far or how close you are to the system limits.