I used to work at SEA for Alaska and I would have to say that there are approximately 120-140 departures depending on the day of the week. During the summer, the flights dramatically increase as SEA is a key gateway for flights connecting to cruises (vice versa). All Alaska flights operate from 15 dedicated gates and 3 gates shared with codeshares.
On a busy day, there are approximately 17,000 enplanements just on Alaska.
I would estimate Horizon has approximately 200 departures. Maybe someone in the forum who works for QX can provide the gate info.
Hope this helps.