I have setup the footer of my invoices. [disengaged custom templates and activate customer footer.
this works well. I added custom fields that are available such as due date and total amount to show up in the text of the footer.
However, I also like to display the invoice number [autogenerated nr] in the footer. below are the text lines of the footer: [dutch sorry] but you can see I already added 2 custom fields to it. and that works just fine 
so I am missing the invoice number itself. If I look at the customs fields, it doesnt show [only salesordernr, but that is a diff field]
U wordt vriendelijk verzocht het bedrag van @@InvoiceAmountInvoiceAm@DueDateunt@@ voor @@DueDate@@ over te maken op IBAN bankrekening:
NL46 ASNB @Vi[deleted by moderator] t.n.v. @Vi [deleted by moderator, do not post private information on a public forum], onder vermelding van het factuurnummer. [here should the invoice number be]
De algemene voorwaarden van @Vis[deleted by moderator, do not post private information on a public forum] zijn van toepassing op al onze leveringen van goederen en / of diensten zoals gedeponeerd bij de kamer van koophandel onder nummer NL 3[deleted by moderator, do not post private information on a public forum]. Deze condities worden op verzoek vrijblijvend aan u toegezonden.