When I'm working within a days travel, I carry 4or5 - 8x20 and 12 - 8x10 holders in the Jeep. When I'm on a trip I'll carry 6 - 8x20, 25 - 8x10 and a Harrison changing tent.
I'd recomend a Harrison changing tent. For about the price of a new CFH you can reload during the day without heading back into the darkroom. It also keeps you from being locked up in darkened hotel bathrooms at night on trips.
Most days I'll only make one or two 8x20 exposures but it's still better to carry more holders then to see something and be out of film. Since buying the changing tent I've only reload the 8x20 holders once on a trip during the day.
When I used to shoot 11x14, I would travel with 4 holders outdoors. In the studio I shot with 8 - 11x14 holders.