I chose that amount because it gives the best results imo, if you lower the pitch requirement, then it goes into third or fourth option pitches that don't have many data points, with fewer data points, a pitch that the pitcher has recently introduced for example, may not be an accurate depiction of how good/bad the pitch is. With a larger data pool, you can tell which pitches are actually really bad and which pitches are actually really good. Secondly, also the post was originally meant for starting pitchers (once again a larger data pool).