Phone numbers in most countries are definitely not scarce. US numbers can be obtained in virtually any area code (excluding 212 and a few more) from between $0.40 and $1.00/month. There are even services that provide temporary throw-away numbers.
Yes but you are ignoring the overall picture. If you factor in all those numbers that can be obtained, it is still de facto close to 1 number per person on average. We are talking about numbers for which an SMS can be successfully delivered and the end-user gets some unique code. You'll find all of those "throwaway" SMS services re-use the same numbers, because they are scarce.
I challenge you to produce any country (link supporting what you say) where phone numbers with SMS DELIVERABILITY are not scarce eg dont need a device or endpoint to be registered with some carrier or can be obtained by the hundreds.
US and UK numbers with deliverable SMS are under $0.40/month each, and can be ordered in bulk for virtually any area code through many providers such as:
