If you ever pay through Apple Pay at a store and check your receipt: The last 4 digits of your card will be shown on that receipt. They will be different from your physical card (unless Monzo does something highly unusual - I haven’t actually checked this with Monzo specifically, but all my other cards behave that way).
TfL contactless doesn’t simply rely on your card number to tell cards apart, anyway: Even if you have two physical cards with the same number (e.g. 2nd card holder for a credit card), they can still tell them apart.
If you want to go really deep into the rabbit hole, this is a good starting point: Curve chat - #264 by GalaxyMergirl
That’s fair enough - different people have different approaches 