Common problems found on agistment proprties include

Non payment
Dumped animals (owner gets behind in payment and never shows up again)
Animals damaging fencing
Animals damaging themselves and requiring urgent medical attention and the owner is unreachable.
Random people coming on and off your property at all hours
Agisters "borrowing" your hoses, shovels, buckets etc.
In the case of multiple agisters, "borrowing"other people's equipment.

Horses will likely need much more day to day maintenance from owners, therefore it is likely you will have people in your private domain regularly.
They are also much harder on pasture.

Cows are much harder on fencing and it can bite pretty hard to convince 20 of the buggers to come back home after they have bolted through a fence.

Depending on your area and facilities, you would expect to charge from $20 per horse per week for a shared paddock with no facilities (arena, wash bay, tack room) and the owner does full feeds and rugs, all they way to $250 for stables, indoor arena, you feed, rug and take care of everything, the owner just turns up, saddles up and rides.

No matter what you charge, it will be expected that you supply water for the stock to drink. Washing is a different issue if water can be tight

I would allow,for fairly good grazing, 1 horse per 2 acres.
That should ensure the place doesn't get trashed.

My two cents, ring up you local feed stores, find out what forage grows and sells the best in your area, and make hay.
Far better return, and no pesky 14 year old girls fighting with each other and hanging around your house all weekend