Actually, I Just looked at the past 6 Truck races, 3 this year, and the last 3 from 06.
Only Half of the past 6 have had full 36 starters, and only 1 this year. So no, not "almost all" races are full.
Plus I did a little game of how many trucks did less than 10 laps.
2, 1, 1, 1, 1, 3, including one who didn't even start. Notice a pattern?