The invoice/credit note numbers are set by our auto-generated numbering system.
You can customize the numbering by setting a prefix for all your invoices/credit notes. To add a prefix, go to Settings >> Customer-facing essentials >> Invoices and credit notes, provide an Invoice/Credit Note Prefix Number and hit Update. Here's a screenshot.
For example, if you set 1500 as the invoice number prefix, the invoice numbers on your site will be 1500001, 1500002, 1500003 and so on.
Furthermore, you can have the invoices/credit notes start from a particular number, we can do this from the backend. For more information on this, contact email@example.com