Hi @Ethan Wang - Wisdom Group ,
After doing several research, there is no Microsoft official document mentioned the limit of users who can have send as permission to a particular mailbox.
So, to clarify the situation, could you please provide more information, such as error messages?
As Vasil Michev mentioned, you can try to add the send as permission by using PowerShell.
Here is an article for your reference: https://technet.microsoft.com/en-us/library/749cdfe3-496b-453f-96eb-20a0bf28fd52
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