Invoice numbers are assigned automatically:

1. Create an invoice; MyCompanyDesk numbers it sequentially, the way the Dutch tax authority expects
2. There is no numbering format to configure
3. Need a specific number on a draft? Open the invoice in the form and click "Set custom number" in the sidebar
4. To start the sequence over, go to Settings → "Account opzeggen" and use "Reset invoice numbering"

Tip: Drafts do not have a final number yet. Delete test drafts before finalizing and your sequence stays without gaps.