> Assuming the last digit is a checksum and the first six are taken by the routing information, that leaves them 9 digits or a billion possible numbers per credit card issuer. Japan has a population of about 125 million. Are Japanese people cycling their numbers so frequently? Or are they big on ephemeral card numbers?

Credit cards - and payment methods generally - are in their Cambrian explosion phase right now in Japan, particularly given the pandemic. Every company, big or small, is pushing their own. I recently had to open a new credit card because the gym I wanted to join only accepts payment via their partnered credit cards. It came with a linked electronic money card (as well as having native integration for a different electronic money format) that has what looks like its own 16 digit credit card number, presumably for internal payment infrastructure reasons, and an offer to apply for a separate linked credit card for shopping in China, three other kinds of linked electronic money cards (one for a supermarket chain, one for a local transport network)...

Every shopping mall is pushing their card. My phone provider offered two olympic tie-in cards and a regular version (that would actually save me money if I could face going through the application). A theatre troupe I follow has a deep partnership with a card issuer and has their own branded cards. Bands have their own cards. Virtually any outfit with a loyalty/membership card is trying to turn it into a credit card. And so people can easily have multiple cards that they never use, because their loyalty card became a credit card but they're still only using it as a loyalty card.