converted discussion to question.

to get number of entries in PP control use following formula

If(Array.isArray(PPControl),length(PPControl),If(length(PPControl)>0,1,0))‍‍

‌ ‌ number of items‌

Hi Marek Skalinski‌, were you able to resolve your issue?

Yes, Thank you, your formula was really helpful

Br

Marek

that's great.

please mark correct answer.

Dear @emha ,
Please help us understand a bit more.
where to input this? is PPControl a named people picker control or not ?

Please give us more information ..
I have the same question but I don't understand your solution.

Thanks for your help

I checked and found it's actually name of the PeoplePicker controll that we set.
Thanks @emha 

This worked perfectly in my test environment. I created a calculated field with the formula to return the number of entries in my people picker control and then created a validation rule for the people picker control to fire if the calculated field  is <> 1. Great.

But, when I tried to replicate in my production environment, the Customer Name (people picker) control disappears from the form and an error appears that 'object does not support property or method 'isArray''.

Any thoughts or suggestions?