Click on Settings.
Then on Invoice Customization.
You’ll see the tab reading Start Invoice Number.
The start invoice number is a number that you should pick as soon as you start your account.
For example, if your start number is 44, the next invoice you create will be number 45.
You can change the number of an invoice by selecting an invoice and clicking Edit, if you put in a number that already exists, it’ll let you know.
It’s important not to edit these numbers too much, for accounting reasons, if you do have missing invoices, it might help you see what happened to what invoice.