You _can_ use autonumbers to save a little effort.
I have an app where job tickets are numbered sequentially starting from 100000. I began by appending 100000 to the autonumber field; when you do this, Access resumes the numbering from the next number i.e. 100001.
When a user wants to create a new job ticket, their button click adds a new record to the table, and then displays that (job ticket) autonumber on the data entry form, filtered for that record. Since I do not let them delete the record, or add new records by themselves, the number sequence is unbroken (bad job tickets are marked 'void' but they stay in the system).
Without the autonumber, I would have to store the last number used somewhere, or search the table for the next number, but my way is faster & easier.