Invoice numbers on documents can be generated by including a simple merge tag on the document template, then generating a Saved Document within an event.  

There are two ways to include an invoice number on your documents, either starting at the number 1 or you can start at your own pre-determined invoice number.  Each time you save a document within your events, the invoice number will increase by 1.

1. Starting your invoices at #1

Create an invoice document by going to Setup -> Documents -> Add New or Import a Shared Document -> then include the merge tag <invoice_number> on your document template.  Next, go to an event in your system, then to the Documents tab.  Under the Saved Documents dropdown menu, find your invoice template and a new page will open.  You will see the number 1 where you included that <invoice_number> merge tag.  Now Save the document.  Now, every time you need to generate that invoice as a Saved Document in an event, the invoice # 2 will increase in increments of 1.

2. Starting your invoices at a custom number

You can start your invoice numbers at a pre-determined number.   You will create your invoice document template the same way as above, using the following combination of merge tags as a calculated value.  For example if you want your invoices to start at 400 you would include this calculated value on the invoice document:  <calculated_value><invoice_number>+399</calculated_value>.  When you save the document for the first time in an event, that document will have an invoice # of 400, and increase by 1 for each consecutive document you save.