How can I have a number field behave like the auto number function?

I saw somewhere that through code you could have the number field increase each record by 1.

Any ideas?

It is often done using a DMax:

NewNumber = Nz(DMax("NumberField", "TableName"), 0) + 1

Where would I place the Code? In the actural number field? In the Form?

Sort of depends on the specifics of your application. If you search on DMax here you'll probably find several discussions on this topic. It comes up pretty often.