I don't think it's defined anywhere.
I would say the reasonable limit is 2 for a single size difference, double for each additional size difference. i.e. Medium on large (human on horse) is 2. Small on large is 4, on huge would be 8, on gargantuan would be 16, on colossal, 32.
2 people on a horse is reasonable, more is pretty much not. 4 human children on a horse is pretty reasonable. And double for each size category pretty much goes with the way size and weight work in D&D.
-The Souljourner