Automatically Numbering a Field in a Form

Hello all. I am developing a form in which i will place multiple entries into one particular table that has Two fields as a PK. The numbering of these entries should be sequential and always begin at 1. The number field in the table is not autonumbered and i have several entries that range from 1-16.
It is easy enough to enter a sequential number for every record i wasnt to submit, but I would really like to know if it is possible for the form to autogenerate the next sequential number for me.
Can anyone give a helping hand? Thanks.