"Feed me! Clothe me! Pay me!" Always me, me, me, never a thought for their poor monarch who has three palaces, a mistress, and a wife. Oh, the travails of being king (Mel Brooks notwithstanding).
Actually, supply rules can be as complex as you can stand. Let's see, this is the 18th Century, so for each combatant figure one hanger-on. Officers will have two to seven non-combatants that they support, and then we get to the general staff. There should really be a separate supply train just for the general staff. You never know when you're going to have to entertain all sorts of people. Hmm, and then there's the support staff, the necessary jugglers, acrobats, and so on. Even the Ladies of Negotiable Virtue (did you know the Croix de Guerre was awarded to two ladies of a Mobile Field Brothel for their activities in the French Indochina War?)
Let's say one man consumes one unit of food and one unit of water every day. An army of 50,000 would consume 50,000 units of food and 50,000 units of water every single day, whether they are in camp or marching. But then there are the horses. it turns out that the 18th Century average for a 50,000 man army was 30,000 horses. it's not just the cavalry, it's the artillery, it's the ones pulling the wagons, and so on. A horse consumes roughly ten times what a man consumes. A man can eat meat, a horse can't, but let's skip over that. And they both drink water. So This 50,000 man army needs 350,000 units of food, and 350,000 units of water every single day. This explains all of the campaigns in the river valleys of Northern Italy and those in the Low Countries. That's where the food and water were.
And I haven't even touched on the hangers-on, lackeys, servants, wastage, and so on. They would often destroy more than they would eat. No wonder soldiers were perpetually short of food.
When I do the calculations for a 50,000 man army, I come up with the food requirements of China. The spreadsheets to feed, pay, and clothe these people would be a game in itself. So let's try the Freddie the Lesser game system approach.
A Simpler Way -
A stack of counters can be fed out of a depot or a city if they are within six movement points of that source. The line cannot travel through an enemy occupied hex, so you can't supply an army from a besieged fortress (I saw someone claim that he could in a different boardgame). Now if that depot is within six movement points of a city, it can supply troops six further points away from the depot, and so on. You can have a line of depots stretching across the map, provided they all are connected to a depot (that's connected to a depot, that's...) that sooner or later connects to a friendly and unbesieged city.
That depot must be garrisoned by at least three strength points. Someone has to work the ovens and so on.
To establish a depot, a force of at least ten strength points spends an entire turn in one hex doing nothing but establish the depot. The beautiful thing is you don't need a general in order to not move.
Attrition from Supply
Oh my goodness, something has happened to the supply lines. If this was the Napoleonic period it would probably be that Corsican fellow. But it's not. So we get to have rules about what happens when there is no food.
Simple. Every stack loses one strength point per turn when the stack is out of supply. This is actually fairly benign, but it suggests keeping all of the men in one place. Independent columns are subject to the same thing.
Let's add that you cannot roll for the conclusion of a siege when the besiegers are out of supply. Fighting battles to interrupt a siege was very 18th Century.
But Wait! There's More!
So let's stick an addenda into the movement rules. Something to encourage people to spread out a bit so they can move, because moving 50,000 men and 30,000 horses down a single road is almost impossible. Think traffic jams, for starters.
So, if a stack has more than, oh, 30 strength points in it, it has one less movement point available from whatever it rolled. This modifier is applied after the die roll and checking for attrition. If it has 60 or more strength points, it has two less movement points available, again, after the die throw and checking for attrition.
So, march divided, fight united. This sounds Napoleonic, but it was obvious much earlier. Marlborough used it during the Siege of Lille.
So that is the basis for the supply. Astute observers will note that I spoke in terms of movement points, not hexes. Supply lines through rough terrain will be shorter, and they will be much longer on roads. That's the idea.