How are numbers given?
From your Invoice number prefix in Settings → Business: "Numbers are assigned when you issue an invoice." The prefix may use letters, digits, dashes and slashes, the characters the AEAT accepts.
What if I made a mistake on an issued invoice?
Issue a credit note (factura rectificativa) against it. The original keeps its number; the credit note gets its own number. Then issue a new invoice with the right details.
Spain is bookkeeping only for now
Until VeriFactu is ready, Deline does not issue Spanish invoices. Invoices you issued in your invoicing software are recorded in the books with the numbers they already have.
Explanation written from the Deline app's own screens and texts (build eba146497 (Deline cf723fe1f)); it has no steps to check.