Are you 100% positive on that? Because last time that I saw that number in my family’s old hotel I remember it being “a virtual number” rather than the customer’s real card.

https://partner.booking.com/en-us/help/policies-payments/pay...

The fact that it stores your card is pretty much an unescapable reality. Just like car rentals, they reserve the right to charge you before, during or after your stay, for the booking or for damages.