Im with you there. When I first saw the number, I thought the 300 might refer to the number of complaints or allegations since priest abusers not uncommonly have multiple allegations.
Ive read and reread this a number of times and see nothing that supports my initial thoughts about this in the article however.
For frame of reference there are around 200 active priests currently in the Diocese of Pittsburgh, which is much larger than any which surround it.
Even if you go back and say there were 400 when most of this was occurring, then yes 300 priests over six dioceses seems like a stretch.