Generally they sell the train front to back. Thats why there are usually fewer in the first couple of cars. Seems like there are a higher percentage of premium car seat tickets sold than standard coach which brings me to the idea that they need another parlor car for each train.
Sometimes the buyers all want first class, and they sell out faster, cause they are "Special". We used to see this same scenario on mainline fantrips, and the first class cars seemed to always sell-out, and sometimes they would try to rent more. Hard to lease on the C&TS though.
I think the Sunset trains are going to pick-up in popularity for the general patrons as they offer the meal and other perks.
Greg