To create an invoice number, go to [Settings] > [Invoicing] > [Number Templates]. Click on [+] to create a new number template.
Fill in the fields and click [Create Template] to create the new numbering.

At the bottom, you have an example of what your numbering will look like.
- The Prefix and Suffix are fixed in the invoice number.
- The field Next Auto Number is the current invoice number. If you are starting new numbering, set this to 1.
- The field Max Number limits the maximum number your invoice numbering can reach. If you set this to, for example, 100, then your maximum invoice number will be 100.
- The field # Auto Number Digits determines how many digits your numbering may contain. If you set this to 3, your automatic numbering will be composed of 3 digits, for example, 001.

You can read how to set the invoice numbering as default in the article:
Invoice presets