Under Verifactu the series matters even more: the invoice number and series identify each invoicing record and travel in the hash chain, so gaps and duplicates become visible to the tax agency. Define your series at the start of the fiscal year and keep them stable all year long.
For a freelancer or a small SL the practical approach is to have few series with a clear rule: one for ordinary invoices, one for corrective invoices and, if needed, one for simplified invoices or for a second line of business. Many businesses put the year in the series prefix so numbering restarts each fiscal year without confusion. What matters is that within each series no number is missing or repeated, and that dates move forward in the same order as the numbers.
The typical mistake is cancelling an invoice by deleting it and reusing its number, or fixing an amount by editing the original invoice. The right way is to leave it untouched and issue a factura rectificativa in its own series. The series appears in every entry of the issued invoices record book and is the key to reconciling that book with your VAT returns. In kontora each series has its own counter, and Verifactu invoicing assigns the next number automatically.
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