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