I’m answering based on gameplay experience and second edition rules, ok? Unfortunately Modiphius doesn’t ship to my country so I have only the ME free update, not the book itself.
So, Supplies. Love me some Supplies. According to 2E (Core/Worlds), one Supplies contains enough for 4 people to travel for 1 day and costs 10 Coin. Also, one day of travel covers 10 miles as the standard.
In Worlds, depending on how easy or hard the lay of the land you’re travelling in is, the amount of Supplies you need changes. For example, traveling in Iradrum is pleasant, so you spend only 1 Supplies per day and you cover 25 miles instead of 10; but crossing the desert of Khett is deadly and costs 3 Supplies per day for the 10 miles of standard travel.
As any item, 1 Supplies should take 1 carry slot of space. So, yeah, travelling far requires a lot of Supplies… That’s why people need villages on the way to reSUPPLY or wagons, mules, etc.
As always, some tinkering is needed. Since 1 Supplies is enough for 4 people per day, if you’re travelling alone or with one single person I allow 1 Supplies to last a day in Khett, for example. This math however is not fun and should only be used if you and your group enjoy this aspect of the game.
Edit. Also, as you noticed based on Ghost Mountain rules, the use of Supplies and the rules can change based on setting, usually to enhance or highlight some aspect of the world. Supplies in the fantasy setting of Alfheim are used differently from the prehistoric setting of Blood & Snow because in the latter they are part of the drama and your survival is tightly connected to your resources. But in the superhero world of Vigilante City and the sci fi setting Warp Shell, Supplies are mostly ignored because they’re usually not part of the tropes of that genre… But they can be in your game, depending on the tone you want and how much bookkeeping you and your players are comfortable with.
Happy gaming!
Oh, before I forget:
Hank also has this video where he talks about how the simple Supplies system can be used in journeys on boats (or spaceships or anything really).