Without checking patch notes to see if they made any changes on this; I vote for 1 horse farm supports up to X units of horseman. It would drastically increase the strategic value of horse farms because you would need more than one, and would pursue additional horse farms based on how many units you want to field at once on horses.
I agree with this for Wargs as well. I could also see a logic for Metal and Crystal, even though they don't necessarily need such a regulation, just to maintain continuity and strategic value. AI can be given extra unit cap for a cheat if needed.
I also believe in having Metal/Warg/Horse upkeep for units which use them. I oppose it for crystal because it is already scarce and regulates itself. However, it could be added for modder convenience and continuity with very few, if any, items that have crystal upkeep.
Lastly, I have experimented with adjusting the production values and think Wargs/Horses are better distributed like Crystals. Have 3 buildings which produce .25, .5, 1 horse per turn. Again, the AI can just be given something like .1 every resource per turn if need be. I won't cry if the AI gets "1 of each" every 10 turns, because this "cheat" lets them "skimp by" but doesn't give them undue advantage.