In countries where letters are used for aircraft registration e.g. XY-ABC, the AS registration numbers recycle (maybe not immediately but I definitely have seen newly acquired aircraft got "early" registration number such as XY-AMN instead of XY-BAY.
But this does not seem to be case with U.S. (N) registration numbers. Even after aircraft leave the service (e.g. N302AB), the newly acquired aircraft are assigned registrations with always higher and higher numbers (e.g. N1227AB).
I am wondering why is this? Why do letter-type registrations recycle, but not number-type registrations?
Don't know exactly why as I've wondered this before, but it's due to technical reasons.
What will happen after N9999... ?
It would be hard for you to get that number though
With 2500 aircraft if you change them every 2.5 years, after 10 years you have reached 9999. There are some servers which go on 8 years of age already.
Why do letter-type registrations recycle, but not number-type registrations?
Number-type registrations do recycle (e.g. Russia - "RA-nnnnn"). I definitely leased recently a couple of aircraft with numbers lower than the already existing ones in my fleet. On occasion, when the leasing contract terminated and the aircraft went back to AS I got the very same registration number for a new aircraft - if I ordered that within a short time span (i.e. 60 minutes or so).
The question was correct in your thread subject title. This maybe something related to U.S. registration numbers. But it certainly is not related to "number-type" registrations.
Airline-specific registrations are handled somewhat differently internally than regular, country-wide registrations. Hence the difference.