[NCC] How Many USA Prefixes are there?

[Ray Conrad NM2O]

Operating from PJ2T WPX SSB last month brings up a question of how many USA 
prefixes are active. 
 
The PJ2T team worked about 510 USA prefixes with about 8,100 Qs. 
That's over 1/2 of what's available. 

D4C was MM last year, and had about 575 with about 14,000 Qs. 
 
How many prefixes are currently active? This will ebb and flow. 
 
I think the number of possible USA prefixes is about 873. 
 
AA-AK is 100 plus 4 in AH and AL:            108 
K, N and W: (3 X 10 X 24) =                  720 
KH, KP and KL: (3 x 5 x 3) =                  45 
Total:                                       873 
 
Unlike other countries, I don't believe the FCC issues special calls with a new 
prefix very often. W100AW is a current exception. 
 
The K, N and W assume 5 prefixes in each KH, KP and KL are assigned. (this is 
high)  
 
Some active stations weren't on in WPX. 
For example, NM2 was missed: The usually active NM2L hasn't been on recently. 
NM2A and D are active in the SS, but not in WPX. 
QRZ shows that all 26 NM2 calls are assigned. 
 
73, 
Ray NM2O