How to generate consecutive "serial numbers" in forms

We use various forms that visitors fill out on our site. How can we insert a consecutive "serial number" or form number so that each new filled out form has a number (say 100 for the first, 101 for the second, 102 for the third etc) that is generated automatically, not by the person filling out the form. Thank you in advance for any help with this. 

3 minutes ago, JeffSB said:

We use various forms that visitors fill out on our site. How can we insert a consecutive "serial number" or form number so that each new filled out form has a number (say 100 for the first, 101 for the second, 102 for the third etc) that is generated automatically, not by the person filling out the form. Thank you in advance for any help with this. 

You may have to store the submission to google spreadsheet and set the increase number there

How critical is it that each sequence number is different? There are some methods I can think of client-side that would have a pretty low likelihood of duplicated numbers. For example one is time based. Use the date and time to create a "serial number". 20210518184445. They wouldn't be in linear sequence. You'd need to decide if that would be a problem. That brings up a second question. Do you care about the number itself. Size for example? Or where the numbers start?

Then with a hidden form field and some Javascript to fill in the hidden field you'd have a "serial number" come along with the form. The Javascript would require the business plan or above.