How to automatically update and email unique document numbers using Google Sheets and Zapier?

Hi 

I want to make some automatization using Zaps

Some emlpoyees, which have to have unique number of document ( sending outside company)

Thay are using google forms to sent data to shhet

Now I need to update column “B” od the sheet for next avaliable number - and I assume that if nest addes row wil be numer 6 in sheet

1. - system will check last number in previous row “B” Column and add “1”
2. - system will minus “1” from las row number and put it in last row column “B” 

Finally will sent gmail to user “your next official number of ducument is “ XXX”

Now i have problem with selecting “last” row  to modyfi data form column and putting correct number ( can`t make any matematisc inslide) - could you please support me ?

Regards

Bartek