I've stopped counting.. My opinion:
Thursday PM or Monday AM departing the following airport hubs would have the highest numbers EWR, ORD, or SFO.
E.g. ORD-EWR (UA 230) or IAH-EWR (UA 2073) typically has a Thursday afternoon departure on a position large/heavy aircraft so chance of upgrade is immensely higher vs. normal single aisle attracting many more 1Ks. The confusion at the gate as 1Ks can or decide not to queue with Group 1 makes the chaos a little comical.