There are 2 ways to set up the numbering of your invoices:
Manual numbering
No setup needed, you simply add the invoice number manually as you create the invoice.
Automatic numbering
Go to your Settings
Outbound
Create online > Autonumbering
Enable Autonumbering
Pick a prefix, a starting and ending date (e.g. Prefix: 2020, starting date: 01/01/2020, ending date: 31/12/2020)
Add a row to create a new numbering sequence
Pick the template and the starting value
Important:
when autonumbering is enabled, the order in which sales invoices are sent to customers determines the numbering order of these invoices. The creation date of the draft invoice only influences the numbering when sending multiple invoices at once.