Luckily I did the math wrong on the 24 choose 12 (2.7M combinations).
That is simply the chance of choosing the correct 12 words.
You'd still have to find the correct order of them afterwards.
I believe this would add a multiplier of 12P12 (12! factorial)

So 24 choose 12 (2,704,156) times 12! (479,001,600) = around 1.3 Quadrillion (10^15) possibilities.
Hopefully that's correct.