Please elaborate in more details, some screenshots etc.
Are you talking about generating a unique reference number or rather increment a number + 1 each time a new form is submitted?
I personally, generate a reference number based on [CurrentDate][CurrentTimeSec]
for example: PCL04042019151605
List and Library entries already have a unique number assigned on creation ID. If its format is not what you need, then there are a few alternatives:
Most likely other community users have even more ways to implement this.