How many devices can HBO max be used

According to HBO Max support article https://help.hbomax.com/ky-en/Answer/Detail/000001258:

There's no limit on the number of devices you can use with your account, however, the number of simultaneous streamsis limited.

and

The Standard plan supports streaming on 3 devices at the same time. If you have the Mobile plan (not available to new or returning subscribers), you can stream on 1 device at a time.

This article also tells you how to remove a device so it can no longer cause you issues.  Keep in mind that if whoever authorized a device that you removed still knows your account credentials, it wouldn't take them long to reauthorize that device.