How are you identifying the person filling out the form? Do they fill in their name? Or is the number generated before a form is even filled out for the first time? If it were me, I would create the a unique number (instead of a running number) and add it when the form is generated. Then just add this number to the sheet when the form is submitted.
Otherwise, you will probably have to use a webapp and create your own html form, or do like in the video above.