Consider the affine enciphering transformation:

f(x) = 9x + 4 mod 49

on single letter message blocks of the ordinary English alphabet A - Z

and the letter b for “space”.

How many more letters can we add to our alphabet without any

further changes to our encoding procedure?

Would this just be 26 x 2 = 52

52 - 49 = 3, hence 3 more letters could be added?

Thanks in advance!