This question is ambiguous: it needs to tell us whether we can leave supplies alone in the desert, or if the supplies must be minded by someone.

If we can leave supplies lying around, then remember that carrying 5 days’ provisions means that starting out, you’ve got enough to travel for 2 days, travel 2 days back, and leave 1 days’ rations on the ground.  We also should remember that we only need one person (i.e., ourselves) to make it to Ash, 4 days’ travel away.   Lastly, recall that anyone returning to the coast can stock up again and set out again.

So if we think of the midpoint (2 days’ travel) as our refueling stop, and the person going to Ash (me) will need 4 days’ additional supplies to get the further 2 days’ distance and back, then let’s think about this experimentally…

Suppose I take 4 people with me.  We start with 5 days’ rations each.  After 2 days, they each drop 1 day of rations on the ground (so 4 total), I pick up 2 of them (so I again have 5 total), and then turn around and with their 2 remaining days they hike back to the coast.  I take 4 further days to go out to Ash and back to that midpoint.  I pick up the last 2 days’ rations on the ground, and finish the journey home alone.

OK, now what if instead they turn around, and come meet me halfway again?  Maybe I only need 2 companions.  So we start out with 15 rations; 2 days in, we have 9 left.  I take 5 of them, leave them with 2 each.  They head back to the coast, I continue on to Ash.  The night I’m in Ash, they’re back at the coast, stocking up again.  The next morning, I leave Ash with 3 (will have 1 at the midpoint), and they set out again with 5 each.  We meet 2 days later, I have 1 day of rations left, but they have 3 each and give me 1 apiece, and then we trot back.  Okay, that works.

Let’s think about this rationally, though… if I COULD leave supplies alone in the desert, then I wouldn’t need any companions, I could just progressively build up piles of stuff at each day’s waypoint all by myself until I can make the journey given the supplies I’ve left out there.  So that doesn’t seem like a fair interpretation of the question.  So that’s my answer: 2 companions, 3 of us total.