Here is the basics of what you are requiring, assuming your Invoice Template is part of the Sheet and not a Form.
Create a Routine Module and paste this code into the module :
Code:
Option Explicit
Sub PlsOne()
Dim num As Integer
Range("A1").Select
num = Range("A1").Value
num = num + 1
Range("A1").Value = num
End Sub
In ThisWorkBook module, paste this code :
Code:
Option Explicit
Private Sub Workbook_Open()
PlsOne
End Sub
Each time you open the workbook, the number in A1 of Sheet 1 will increment by one
number.
You can edit the code to have this applied to any sheet and/or any cell.