An invoice number is the unique reference that identifies one invoice and places it in your sequence of invoices. For VAT invoices in the EU, the VAT Directive requires “a sequential number, based on one or more series, which uniquely identifies the invoice” (Article 226, point 2). So you can run more than one series, for example one per year or per business line, as long as each number is unique and the numbers follow in order within their series. It usually sits at the top of the invoice next to the issue date, and it's the reference clients should quote when they pay. Example: INV-2026-0001, INV-2026-0002, INV-2026-0003, with credit notes in their own series, CN-2026-0001. Common mistakes: numbering by client or by date alone, so two invoices can end up with the same number, and deleting an issued invoice and reusing its number instead of cancelling it with a credit note. Your tax authority may set more specific rules, so check its guidance.
Read more: How to write an invoice (step by step).