6gen is expected to enter service in 2030s.
As per natural evolution, one day all 4gen F-15, 16, 18 will be retired from USAF & USN. That could be 2050s/60s.
But then what will replace them & in how many numbers?
Looking at Wiki data for a generic reference (may not be accurate), not incl. trainers :
(
List of active United States Air Force aircraft - Wikipedia)
(
List of active United States naval aircraft - Wikipedia)
4gen:
F-15 A/C = 168
F-15 Str.E = 219
F-15 EX = 8 (104 planned)
F-16C = 738
F18 E/F = 421 (+76 ordered)
EA-18 = 153
5gen:
F-22 = 187
F-35 A = 419 (1,372 planned)
F-35 C = 30 (+204 ordered)
Total 4gen in 2040s/50s = 168 + 219 + 112 + 738 + 497 + 153 = 1,887
Total 5gen in 2040s/50s = 187 + 419 + 234 = 840+
So the question is how many 6gen F/A-XX & NGAD, manned + UCAV?
A better question is what type of 6gen - heavy class F/A-XX + heavy class NGAD + medium class NGAD?
Is it going to be something like:
F-15 heavy class (168+219+112) > F-22 heavy class (187) + NGAD heavy class (??)
F-16 medium class (738) > F-35 A medium class (1,372) + NGAD medium class (??)
F-18 medium class (421+76+153) > F-35 C medium class (234+) + F/A-XX heavy class (??)
And even better question - will they have export versions?