I would run code on the form to find the DMax of that field and add 1. You might be able to use a macro to do that, but I don't use them so I don't know. The right moment to do that depends on factors such as whether or not it is a multi-user db or if the numbers must be consecutive. In those cases you don't want userB to start after you but finish first then end up with the same 2 numbers (or your record save raises an error if you make this field unique). Nor would you want cancellation to cause numbers to be skipped. We'd have to know more about the situation in order to provide more focused answers.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.