What is the difference between the maximum number of users supported by an AP and the number of concurrent users?

The number of wireless access users: the number of STAs that access a Wi-Fi network in a specific space, regardless of whether wireless traffic is transmitted between the AP and STAs (this is the commonly known as the 'number of access STAs').

The number of concurrent users: the number of STAs that access a Wi-Fi network in a specific space and are simultaneously transmitting data (this is the commonly known as the 'number of concurrent users').

Link: https://forum.huawei.com/enterprise/en/WLAN-Q-A-Understanding-the-Number-of-STAs/thread/533101-869.