Credit cards numbers are typically 15-19 digits long, and that's actually a lot of numbers. Not all of those numbers are available for card numbers, but each issuer still has 1 trillion numbers. One of the largest issuers, Visa, as of 2018, has approximately 755 million Visa credit cards in the world. That leaves Visa over 1,000 extra numbers per existing card to work with, but only a small percentage of people actually use virtual cards. Furthermore, virtual credit cards are typically short-lived and could be recycled, especially with revolving expiration dates.
To your point though, if every existing Visa card holder started to use virtual numbers on a day to day basis, at some point I suspect Visa would start issuing longer numbers. But I'd guess were many years away from that becoming necessary.