airport # flts (includes codeshares)
ATL = 19
AUS = 1
BOS = 1
BWI = 2
CLT = 3
DEN = 3
DFW = 10
DTW = 5
EWR = 2
HOU = 7
JFK = 11
LAX = 4
MDW = 3
MIA = 12
MSP = 10
ORD = 7
PHL = 6
PHX = 3
RFD = 1
SEA = 1
SFO = 2
SLC = 1
STL = 1
VKO = 1
YEG = 2
YLW = 1
YMM = 1
YQC = 1
YQR = 1
YUL = 4
YVR = 2
YWG = 2
YYC = 2
YYZ = 3

Top non-US airports:
PTY = 12
BOG = 3
CDG = 2
and even Moscow gets in on the act with VKO = 1