Welcome to the forums and the Seattle area.
Probably 8-14 would be the number of people at the tournament, depending on schedules, type of tournament, and other considerations. Our group of players around the Seattle area has changed from highly competitive to a casual, more hang-out type of group. A group of us (myself included) wouldn't place well at anything besides air hockey (I'm pretty good...) and the usual winners of the past DDR tournaments wouldn't dominate unless it was a crazy mod/crazy ruleset that wasn't just perfect/great count type.
There could be a casual tournament with a pop'n/DM side tournament, as more people seem to be playing both of those (mostly pop'n though).
As for the pads, as long as they're in good shape we really won't need to be opening them, nor worrying about pad misses. Our arcade machine vendor for the tournament locations (Bill) keeps his machines in great shape and would make sure they would be clean for a tournament.
Have you visited any of the area's machines yet?