The serial number is a display only field which is automatically allocated when a new record is created.  This number cannot be altered and remains unique to the record.  Therefore, if the record is later deleted, that serial number cannot be used again.

This means that, while the screen may display:

indicating there are 8 records which may be viewed or updated, there will not necessarily be 8 serial numbers in sequence making up the 8 records.

Example

There are 8 records listed.

As some records have been deleted, the serial numbers making up the 8 records are 1, 2, 4, 5, 9, 12, 15 and 16.