Pedro,,
I hunt with my PCP's all the time and a full reservoir of air in my rifles has always been more than enough for a days hunt. I'll spend 1/2 the day, sometimes the entire day, in my favorite woods hunting rabbits and squirrels and by the time I've bagged my game I usually have only taken anywhere from 6 to perhaps 15 shots. There have been days where I've taken 20 shots, but I don't think I've ever taken more than that and like I said, 10 or so shots is usually a good average for me for an afternoon hunt. When I get home I recharge the tank so I'm ready the next time I hit the woods.
If you're sitting on the outskirts of a prairie dog city or something like that where you're going to be shooting all day then yes it would be prudent to bring along an air tank or your pump. But for regular small game hunting where you will be picking your shots a full charge in the gun is more than enough to last for a days hunt.
PeakChicks suggestion of a pony tank is excellent. It's probably the most convenient way to bring extra air with you,,, but a pump will work well also....
Jeff