Auto number Excel file each time its saved

I have a global template at my company using which I am saving SOPs. Each time I create a new SOP with this template, I number the SOP with one greater than the previous SOP number. This process is causing a lot of discrepancies as there are many instances of SOP numbers being repeated or skipped. I would like to make this process automated and robust where in each time an SOP is saved, it automatically takes in a value one number greater than the previous SOP number. Please advice on how I could do this. Thanks in advance.