I goofed on my calculations a little bit -- steno boards are designed so that one finger can depress two keys at once, so in principle you can actually hit all 20 keys simultaneously. You can't just hit any two keys with the same finger, though, so I don't believe it gets quite to 2^20 possibilities.

(Also, I computed only 20 choose 10, when you can of course have any smaller subset as well. I guess the point here is, a stenotype gives you even more than 184k inputs.)