Here's the list, that's to Thomas for the tip on working it out via Apterous lexplorer:
A: 28 9-letter words with 4 As, including my personal favourite MWAHAHAHA
B: 13 9-letter words with 3 Bs, including COBWEBBED, BLABBERED etc
C: 1 9-letter word with 4 Cs: CACHECTIC
D: 1 9-letter word with 4 Ds: DISBUDDED
E: 1 9-letter word with 5 Es: BEEKEEPER
F: 9 9-letter words with 3 Fs, including FOODSTUFF, FLUFFIEST
G: 39 9-letter words with 3 Gs, most ending -ING, but none with 4 Gs (there are 7 and 8 letter words with 4 Gs)
H: 6 9-letter words with 3 Hs, including MWAHAHAHA, CHIHUAHUA and HGHLIGHT
I: 1 9-letter word with 4 Is, PIRIPIRIS
J: 1 9-letter word with 2 jS: VAJAYJAYS
K: 1 9-letter words with 3 Ks: KICKBACKS
L: 3 9-letter words with 4 Ls, HILLBILLY, ILLEGALLY, WALLYBALL
M: 2 9-letter words with 4 Ms: MAMMOGRAM, MAMMIFORM
N: 5 9-letter words with 4 Ns, CANNONING, UNBANNING, UNMANNING, UNPENNING, UNPINNING
O: 24 9-letter words with 4 Os, including COOKBOOKS, FOOLPROOF, POOLROOMS, VOODOOIST/M
P: 18 9-letter words with 3 Ps, including PREPPIEST, POPPYCOCK, PEPPERING
Q: 3 9-letter words with 2 Qs, EQUIVOQUE, QUINQUINA, QUISQUOUS
R: 2 9-letter words with 4 Rs, REFERRERS, WIRRWARRS
S: 5 9-letter words with 5 Ss, ASSASSINS, ASSESSORS, POSSESSES, SASSINESS, SISSINESS
T: 3 9-letter words with 4 Ts, STATUETTE, TATTOOIST, TITTIVATE
U: 22 9-letter words with 3 Us, but none with 4 (MUUMUU, MUUMUUS, TUKUTUKU are 6/7/8 letter words with 4 Us)
V: 50 9-letter words with 2 Vs, none with 3
W: 2 9-letter words with 3 Ws, POWWOWING, WILLIWAWS
X: 2 9-letter words with 2 Xs, EXECUTRIX, EXOTOXINS
Y: 100 9-letter words with 2 Ys, but none with 3 (SYNONYMY is an exampple of a shorter word with 3 Ys)
Z: 60 9-letter words with 2 Zs, none with 3 or 4 (the 7 letter PIZZAZZ has 4 Zs)