An invoice number is a unique label for one bill. Clients and you both use it in email, bank references, and follow-ups. Reusing a number makes records hard to trust.
A simple format that works
Use a short prefix and a rising integer: INV-1042 or IB-2026-018. Keep the prefix stable so you can sort files. Do not skip around without a reason; gaps are fine, duplicates are not.
What to avoid
Do not use only the client name. Two jobs for the same client need two numbers. Do not use the amount due as the number. Do not restart at 001 every month unless you also include the year and month in the prefix.
Where it sits on the page
Put the number near the top with the invoice date and due date. Every complete invoice should show it clearly.
InvoiceBizz starts a new form with INV-001. Change it to match your own sequence before you download the PDF in the free invoice maker.