No USDA regulations, though your land might come under some local zoning restrictions. On average, "they" say you can keep five sheep to the acre, but that depends on what's growing on that acre, and whether your climate allows it to grow all year. (Zone 4, probably not, so you'll want to lay in some hay for the winter.) I'd ask the local extension agent to come over and help you assess your property with regards to keeping small ruminants.