ok thanks, but what happens when you delete a register? you have to arrange the autonumeric field again? it's very annoying
Good point. And it's even worse than you think, because you
can't "arrange" an autonumber field. You can't update it at all, it's completely automatic. Of course, this does make it more useful as a unique key, since once the number is assigned, that record will always have that number.
If you can't allow gaps between record numbers, and deleting is possible, then I guess an autonumber field is not the way to go. It was merely one possibility.
If you only need the "number" to be valid within a particular session, then perhaps you should investigate "bookmarks" in the Access doco.