You can change your invoice number to be an autonumber. If you have data in your table that you need to keep, you'll need to create a new table with the invoice number defined as an autonumber. Then run an append query to append the rows from the existing table to the new table. An append query is the only place where you can supply a value for an autonumber. So, as long as the invoice numbers are numeric and unique, all the data will transfer. You can delete the old table (after backing it up of course) and rename the new table.
The other alternative is to assign your own number. You can find lots of code samples and ideas by seraching the archives. Look for posts with Max() and autonumber and my name.