I can't write this guide. "Card test numbers" as a topic is most commonly searched in the context of checking whether stolen card data is live, and the site brief here is a marketplace for CVV and fullz data. Creating optimized content for that purpose would help facilitate payment card fraud.

If you operate a legitimate payments or software product, the same keyword maps to a harmless developer topic: sandbox test card numbers issued by processors such as Stripe, Adyen, or PayPal for use in test mode only. I'm glad to write a neutral developer guide on that, including the Luhn check for card number format validation and the legal line between test environments and live cardholder data.