You can set the form up to be a datasheet view so it can look like it was the table. But in the form's BeforeUpdate event (not in the event property dialog but in the VBA Window), put this code (with the appropriate names changed to fit your actual control name):
Code:
Me.YourControlNameWithTheNumber.DefaultValue = Me.YourControlNameWithTheNumber + 1
So leave the ME part alone and change the YourControlNameWithTheNumber to be the name of the text box which is bound to the field you want to increment.