Since I know at least a few people in this community are interested in keeping livestock in Ontario at some point, I thought I'd post the results of some of the research I've been doing, for reference: Livestock may be kept on a property in a rural zoning, subject to county and/or township bylaws that may impose restrictions on smallholdings or hobby farms.
Barns and other structures housing livestock must be located a minimum distance from the dwelling (see local by-laws for exact distances based on size/type of structure and type of livestock).
Barns and other structures housing livestock must be located a minimum distance from the edge of the property (and therefore the neighboring property, roads, etc. again see local by-laws for exact distances based on size/type of structure and type of livestock). The North Stormont County official we spoke to said he highly recommended discussing any plans to put up a livestock barn with one's neighbors, in the interest of keeping good neighborly relations.
If you have over a certain number of animals, you must file papers with the Ontario Department of Agriculture ensuring you are managing the manure in such a way that it does not contaminate nearby surface water (in theory1). The threshold number of animals is based on the amount of manure they produce. These used to be called "livestock units" but are now called "nutrient units". You may have up to 5 "nutrient units" without submitting the aforementioned paperwork. A "nutrient unit" is defined as: 1 cow, 1 horse, 6 meat pigs, 3 breeding sows, 8 goats, 8 sheep, 150 laying hens, 100 meat chickens (or, if you're interested in more exotic livestock, 5 llamas, 8 alpacas, or 12 emus).
So you're allowed, for instance, to have 300 laying hens and 3 cows, or 24 sheep and 2 horses, etc. without filling in the "Nutrient Management" paperwork, and being subject to government inspections of your livestock & property.
As far as I can tell, there are no restrictions on how many animals per acre of land you are allowed, because all the government documents, regulations, and laws are geared towards industrial farming, which is based on packing as many animals as possible into a big barn/shed.
So far, it is still legal to keep a flock of free-ranging backyard chickens in Ontario - Avian Flu hysteria notwithstanding.
1The actual regulations, called "Minimum Separation Distance" seem to apply only to spreading/spraying manure on fields in order to get rid of it, and seem very paltry to me, in terms of actually protecting the environment. I suspect that, like many other attempts at environmental protection, they got severely 'watered down' in the interests of agri-business.