So for each distinct sentence, we pick randomly from the n different hashes, and we attempt to brute force this n times. So the probability of not getting this is:
(1-1/n)^n
As n increases, this will yield e^-1, so we have about a 36.7% chance of this not happening for any given length. So this has a probability of happening of 63.3%.
So there is a decent chance that there exists a sentence "The sha256 of this sentence is ..." for even the full sha256. If you are allowed to modify the sentence to be something like "'Begins with', 'starts with', 'OMG guys, check this out':" you can get this up to almost 1. Finding it would be mildly hard though barring some novel discovery about sha256.
