Welcome to the Community, qb13.
You've come to the right place for assistance. I'd be glad to show you how to view the highest invoice number used.
Just follow these steps:
1. Hover over the Sales tab in the left navigation panel, and click Invoices.
2. When viewing the Invoice list, select the column header titled Invoice to change the order.
That will do it. With this info you'll be able to see the last and highest number used to identify an invoice. Please feel free to reach back out if you have any other questions.