Is there a particular point of time or 'event' when you want it to increment?
E.g. when you go to print? If so then the incrementation is quite simply
added as part of the code that runs this. Even if you have recorded the
macro, you can simply add as a last line before the 'End Sub':
Range("B2") = Range("B2") + 1
Substitute the range that represents the cell where your invoice number lies.
Alternatively you can attach this code to a button etc.
Helpful? Then click Yes.