Two, a power tank and a tire plug kit. Have used both spares, plugged one and borrowed one off the chase trailer before on a run. I have seen guys get flats as soon as we hit the trail head. So two are needed if you want to venture very far into the desert.
I also read on here before, just carry one and borrow a buddies tire if needed. That's all well and good till someone ruins your $450 borrowed tire. Best bet is always be as prepared as you can be to fix your own truck.
x2...