N8634A is taken. It's currently assigned to a 2014 Boeing 737-8H4 registered in Dallas, TX.
See the full aircraft record →A tail number only frees up if the owner deregisters the aircraft or moves it to a different N-number — and even then the FAA holds it before releasing it to the public.
N8634A has been worn before. Past registrations from FAA deregistration records:
Tails one nudge away from N8634A that aren't assigned or reserved: