bolded letters are used more than once (bolded so that they aren't counted more than once)
cut down the middle:
A, H, I, M, O, T, U, V, W, X, Y
cut across the center:
B, C, D, E, H, I, K, O, X
rotational:
H, I, O, X, Z, S
So the total list:
A, B, C, D, E, K, H, I, M, O, T, U, V, W, X, Y, Z, S
18 letters
edit: oops i didnt add BCDEK ;D