I wish I was able to track more frequently than every 15 minutes (free version api max, etc), because some aircraft pass overhead before they're picked up, so it's not the most accurate, but a rough figure to/from O'Hare, Midway, and Gary
... for my project, I actually got some historical paper schedules of the official aviation guide, basically they’re phone books. I hope to find a decent/affordable database for more recent data. (MIT students/alumns actually get access to a database going back to 1979, but alas no access for outsiders...)
The actual seats bit is surprisingly complex if you want accurate figures, as the same aircraft type can have wildly different numbers of seats depending on layout and class configuration. OAG/Innovata's standard schedule product has the aircraft variant normally assigned to a route shown, and they survey the airlines on the seating configurations of their aircraft calculate capacity and ASKS. I believe Cirium now cross reference this with flight tracking data to get data based on the actual aircraft used (which solves edge cases like substitutions or an airline operating aircraft with differently configured A330-200s on different routes) - doing that was part of the masterplan when I worked for them before they acquired Flightstats.
You may already be aware of this but if you want real-time ADS-B, check out PiAware (https://flightaware.com/adsb/piaware/) as a low cost option to run your own ADS-B ground station via a raspberry pi.