Is this available to SaaS users?  I can't see how to enable developer mode or get to technical settings.

Hi,

Invoice number that is sequence will be automatically assigned based on the sequence. If You haven't created any invoices yet(ie, no any validated invoices), you will have the option to edit the invoice number. Once you edit and assigned an invoice number next sequence number will be based on the input value.

You can see the sequences from ,

Settings -> Technical -> Sequence & Identifiers -> Sequence