I'm content with a single supply cart, but I would like to be able to load it up with more than 25 000 supply.
At 2nd Bull Run/Manassas. the supplies do not reset between the days, and even with 2 corps with 2 supply carts and 25 000 supply, max logistics and corps commanders with logistics AND a few captured union supply carts, I could not keep my troops supplied - half the army ran out of ammunion before the first day was over. And as I was not able to keep the captured supply carts at day 2 (I captured them late day 1 and did not have time to bring them to my entire line), the half load left in my 2nd Corps supply cart could not sustain the army for day 2.
I'd suggest to make the supply carts able to keep as much supply as you like, but the cost for each 1000 supply to increase as you buy it, making it more and more expensive the more you expand your supply.