Here's a recent post from John W. Vinson, MVP, on this subject that you ought to read:
“When using Autonumber, do be aware that there will be gaps in the numbering - any record that's deleted will leave a gap; hitting <Esc> after starting a record will leave a gap; adding records using an Append query may leave a gap, often a huge one; replicating the database will make your invoice numbers random.
In short... it's best NOT to use Autonumbers for human consumption, and particularly not for consumption by accountants and auditors. Invoice sequences like 319, 321, 322, 385, 386, 221841246, -1083225152 make such people get very nervous.”
If you need consistently consecutive numbers, you need to generate your own incrementing numbers.
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