SUPPLY is a restraint that you impose on your players to control how far they can move on the world map. I use it when I feel like such a restraint would add to the game. If you do not feel like it adds anything to your game, it’s absolutely fine to discard it.
With the disclaimer out of the way…
Your frustration with SUPPLY might come from one of two things: your players are not doing their part or you have yet to untangle the mess that is SUPPLY in ICRPG. It is a misunderstood mechanic and I’ll get into it a bit later.
While your job is to adjudicate the actions of your brave/foolish/greedy/heroic adventurers, you cannot track everything. That’s why players record things on their character sheet. It should be up to them to be aware of how much supply they can buy and have before taking the decision to go on trip.
You could be a bit more strict with them about this or you could shift the responsability over to you if you feel able to do it. Pool their resources and substract them as time passes, you will learn to remember it in time.
Tracking resources doesn’t need to be fun to facilitate the generation of fun events during your game. Dilemmas can arise from your players finding a promising ruin but argue about going in or continuing their trip to save up SUPPLIES.
Tallying SUPPLIES might not be fun in itself, but it doesn’t need to be complicated either to be useful. I think that there is the issue of untangling how SUPPLY works exactly and how it might cause some unnecessary headaches to DMs.
To my knowledge, ICRPG 2E never really mentions SUPPLY as a mechanic. It’s present on one of the character sheet but does not seem to be important until the WORLDS book. At that point, I think that the whole community had been exposed to the fact that ICRPG mechanics are malleable. Relative to the setting in which they are used. As such, I think that it’s logical to assume that people have misunderstood “four people per SUPPLY” introduced in Ghost Mountain. Besides, it would seem weird for Runehammer to introduce fractions in ICRPG.
The original idea seems to be obvious: spend X amount of SUPPLY per day of travel, according to the region your character is going through. SUPPLIES seem to be a mix of food, water, antiseptics, and bandages. There was the issue of how much SUPPLY a character could hold in this thread but I think that the answer should be fit the spot that SUPPLY occupy on the character sheet: it is a score (a stat) and as such should not rise beyond ten.
Speaking of my personnal approach to SUPPLY: I allow players to spend one during a fight to heal 1D6. Since there does not seem to be an exhaustion table in ICRPG, I either use the one from D&D 5E for inspiration or make my own depending on the setting. Otherwise how can you tell when a character is about to starve?
At the end of this post, I’m going to shill my own Supply Heart rule if you want to check it out.
If you have other questions about SUPPLY or want to discuss further, I’d be happy to brainstorm more with you! In the meantime, roll fun! o7