Paperform already provides a unique identifier for each submission received (the Submission ID). If you'd still like to produce a number for each response then this can be achieved via use of a Calculation field, although we don't make guarantees that this ID will be truly "unique".
Calculation fields are available with Pro & Agency plans.
This calculation will generate a number using the current date & time, which can be found in the submission results. This means that if more than one respondent submits your form in the same millisecond, they will receive the same number.
Note that the same is not true for the automatically generated Submission ID included with every form response - these will always remain unique.