I'm pretty sure Jack was alone the first summer.
Aguirre's words:
"Bad predator loss, nobody near lookin after em at night.
[...]but the HERDER [...] sleep with the sheep, hunderd percent [...]
Got the dogs, your .30-30, sleep there. Last year had goddamn near twenty-five percent loss, I don't want that again."
I don't think there was a herder and a camp-tender the year before. Jack was all in unison, thus didn't sleep with the sheep at night, thus 25% loss.
Once a week someone came up with supplies, I guess it was the same one year earlier. With that, Aguirre would have fullfilled his duty of care. I think in 1963 it was an acceptable duty for a young man to spend a week alone in the wilderness.
Maybe he also sent a second guy for a day or two when the herd needed to be moved between pastures. But other than that, I think Jack was alone up there.