Hi, @ameneh maleki ,
Per my test, we can use the count() function in InfoPath to count the entries in the people picker field.
I created a demo list form below to demonstrate:
The rule of the button is set like below. Notice, you need to switch to advanced view to find the Display Name field.
For some reason, I found that before inputting anything , this rule will return 1. However after inputting an entry, it works properly. The function will return 0 when I deleted all entries. You can do some more tests about it.
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