That is not very much (pretty normal) and hashfails do not affect the final download.
A hashfail occurs when the content is corrupted in transfer. The torrent file only allows your computer to accept content that is in accordance with its hash. This ensures that your final download will be uncorrupted.
It's just telling you that it had to re-download 20 pieces, totalling XXX Mb
So the hashfail has nothing to do with the original content of the file being OK or not. It is data that was rejected because it was not identical to the original content. So don't you worry to much now...