AutoNumber will develop gaps!
Form example, when you start adding a new Record, JET will allocate a value
to the AutoNumber Field for this new Record. If, for some reason, you
cancel the addition, the allocated value will NOT be recovered and the next
new Record will (usually) be allocated the next value.
If you referred to the number in the Navigation Bar of your Form, that is
NOT the value number of your Record. It is actually the RecordNumber of the
Record in the Form's Recordset. Note that the AutoNumber value of a Record
is fixed but the RecordNumber depends on your selection criteria and the
sorting on your Form. Try sorting your Form Descending and it is likely
that the RecordNumber of each Record will be different from the AutoNumber
value.