Cities in the theater can supply about a division and a half. You can purchase few more wagons, but anything more would require too much depots and would be too expensive.
Most of the supply moves along the stockades from eastern Kansas toward New Mexico.
If you play the Union in one of the grand campaigns you'll quickly notice that the South is trying to grab a line of Union stockades from south-west of Fort Smith AK toward Denver. If they succeed you'll get very little to no supply into New Mexico after that, unless you succeed in building a roads with depots from southern California, which you would have to start very early, because it will take a very long time to complete.
The South is pulling its supply from Texas, which is practically impossible to block without invading Texas.
I was noticing in my game as the Union against the AI, Largeish confederate forces getting as far as Denver, CO before starting to go unsupplied. If the AI can do that then I would worry that a human playing the CSA could mount a division+ scale offensive across the plains of Texas, Kansas, and Colorado. That seems less than plausible.
Theoretically the CS player could do that. I've already seen Athena sending one of those fat Texas brigades into New Mexico. The problem will be keeping in keeping large numbers of elements supplied even if they are sitting on the supply line.
Also the player should be aware that if you assault a stockade there is always a chance of it being destroyed, and then you'll have a chink in your chain of stockades.
Just that we are talking about the same thing, these are stockades:
event if they they get named "Fort" so-n-so.
When I look at them, especially the ones going from Counsel Rock to New Mexico, they almost all have between 100 and 150 GS, and that's not being generated by itself.
Look at the supply overlay, and see if they send anything. They are good supply pull though if they are in a range of a depot. Forts/stockades would pull as much as a supply wagon.
I'll have to take a closer look and keep track, but between Denver and Counsel Bluffs there are no depots north of the Red-Sabine Rivers. If the stockades were not moving supplies, then they would also not be stockpiling it, because there is nowhere from where it might be taken.
Besides, as Pocus just confirmed, you can only exert supply-pull over at the very most 5 regions, depending on terrain and weather.
So, it would have to be level 10 to send anything, so no sending, only drawing if there is a depot, lvl 3 city or lvl 5 harbor within 5 regions radius.
On other note, if there is no consumption, within 2 years, a stockade can fill 100 GS from its own production.