Typically, District events shoot for 40 teams.
It's a number that evenly divides by 12 matches per team and generally fits into the smaller venues used.
40 teams * 12 matches per team /6 teams playing per match = 80 match event
80 Qual matches ~= 9.5 hours of play.
Some venues might be smaller, because they may not have enough space for spectators or pits.
Events can always add that extra rookie or veteran to accommodate special circumstances.
You can see last year's district event totals
here.