Based on every single airplane I've ever seen or been on, with bags I'd say about 0.8 times an many people as there are rows, plus about 4 people between the front row and the exit door. On a Southwest 737 with 24 rows, that works out to about 23 people.

I've never seen deplaning be the hold up, it's boarding that takes forever. But getting people to follow complicated high-speed boarding plans is like herding cats.