It needs to be automatically filled in at the time of the initial form submission.  Do you have an interesting idea that doesn't involve Workflow?

Preferably the person filling out the form but if you have an idea for the first task, that might work in some cases.

It would still include Workflow, but hear me out. First, create a database table with a single column as an int type. You can put any number you want as the value for the first record.

Then in your Forms process, after the initial form is submitted your first step would kick off a Laserfiche Workflow. This workflow would first do a query to find the max value for the SQL table column I mentioned earlier and return its value. It would then add 1 to that value, insert that value back into the database table, and then using the new features in Workflow 10, write it back to a field in the submitted form.

It would then be displayed to the person that receives the first user task.

You could have it displayed to the person filling out the form as well by creating a SQL view that brings back the MAX value and displays it back into the initial form, but depending on how many are being submitted at the same time, that same number may be assigned out more than once.

You could have a different table for each form that needs the number or add a second column with the value of form and in your queries from Workflow make sure it has a where clause of where form = '(value)'.