How many max devices under each external user Apple ID for TestFlight

What is the maximum number of devices can be used under one Apple ID for TestFlight ?

We have recently encountered in an issue connected with reaching maximum devices limit - we had about 30 (or a little bit more) devices added to specific Apple ID. After we had removed one of device from the settings of TestFlight - we were able to install update.

How many max devices under each external user Apple ID for TestFlight
 
 
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