How many people can ride one horse?
First you prepare your horse. Get it nice and clean, and try to pick a calm one. Go for a larger size if one is available.
Just one rider? That's not even really trying, even if you ride it sideways like a "proper" lady We can do a whole lot better than that!
Two is still pretty easy. Especially if you use small people.
Three is starting to get tricky bit still simple enough with smallish people and a draught horse.
Using little people and a little horse is trickier... but it can be done. Dude on the front doesn't look to comfortable though.
Five is starting to get seriously impressive. Professional circus performer territory. You'd also need a rather strong and very well-behaved horse.
If anyone has beaten five I have yet to find the photographic evidence.