I want to create an invoice template in Excel, each time I create a new
invoice I will save it as a worksheet/workbook. I need a new unique invoice
number to be automatically generated each time I open the template. I
believe I will need to create some form of database to store invoice numbers
but I don't know where or how to get started with this.

I looked at data - Import External data but this is grayed out?

Can anyone help?

Thanks.