Under Settings > Invoicing within your event, you’ll see the option to use a custom format for invoice numbers.
By default, invoice numbers will simply start at 1 and count up. There’s a field for you to change this to a different starting number if you’d prefer.
Use the following variables to change the format:
{{number}}
— Displays the default invoice number.{{number_in_year}}
— Displays an invoice number that starts from 1 each year.{{year}}
— Displays the year the invoice was created.