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From what I know there is no out of box control you can use. I assume the number is per instance of form. So if I have Form1 and Form2 using two different section site, each of them need to start from "1". To do that you need to create custom form elemnt. See:
http://www.david-tec.com/2016/01/building-out-a-custom-form-element-with-the-new-episerver-forms/
And then behaviour of it is perhaps in Dynamic Data Store to store current "Number" associated to each form. On submit you just need to read DDS value associated to the form, increase it and save it against value of control.
Hope it helps
Is there any possibility to add autoincremental hidden filed that is sent in the mail to admin, and is increased after each submit ?