Earlier quoted context omitted.
It seems like their goal is to maintain a threshold such that x petitions get responded to, where x is a number they can manage and afford. I highly doubt x is actually zero, so they will probably adjust the threshold in the future if needed.
Why not just say "We'll respond to the top X petitions per 30 days" then?
"Top X/month" would assume that they would choose a time of the month when they would compare and respond to those which is probably not the way things work(they respond to things as they get around to them, whenever that is and as fast as possible).