Every order you create will begin in a Quote mode (unless the Support Team has changed the configuration). The number label on your customer invoice depends on the status of the order. If it is in a Quote mode, it will say "Quote #764"
if it is Open "Order #764"
and if the order has been Completed "Invoice #764"
To have an invoice, your order must be "Open" and be "completed" by marking your services complete and rentals returned.
If you would rather be able to have it show the "Invoice #" no matter what, you can create a new Invoice Template and change that label to "Invoice".
When you have accepted the full payment on the order, you will see a red "paid stamp" on your invoice template.