Here's a typical Auto-incrementing Number hack. The first code here would be
for an IDNumber that is defined in the table as Text datatype. "Number"
fields that aren't used for math really should be defined as Text.  

Private Sub Form_BeforeUpdate(Cancel As Integer)
If Me.NewRecord Then
 If RecordsetClone.RecordCount = 0 Then
  Me.IDNumber = "1"
 Else
  Me.IDNumber = DMax("val([IDNumber])", "YourTableName") + 1
 End If
End If
End Sub

Here's the same code for an IDNumber defined as Numerical:

Private Sub Form_BeforeUpdate(Cancel As Integer)
If Me.NewRecord Then
 If RecordsetClone.RecordCount = 0 Then
  Me.IDNumber = 1
 Else
  Me.IDNumber = DMax("[IDNum]", "YourTableName") + 1
 End If
End If
End Sub