As Tracy suggested, it's important to understand that coyotes tend to attack in packs. Although only one coyote will generally show itself at a time to lure the dogs.
Back in CO, our land had a decent sized coyote pack. Actually there may have been more than one pack. Probably an alpha pair, 2-3 older pups and the new year's crop of pups. By the voices we heard at night, I'd say we had about 6-15 coyotes at any one time. (On 40 acres.)
At the time we had 2 dogs (the third dog came around just before we moved back to disgusting FL). Between the 2 dogs we never really worried about the coyotes. On the one occasion we actually saw one (I believe it was the male alpha) a few shotgun blasts in the air sent it scurrying.
One thing we did encourage was a strong pack mentality between our dogs, so that they wouldn't try to take a predator alone. Our dogs were also inclined to stay near us as possible, which helped. The third dog was actually a meditated choice to allow the dogs to triangulate attacks, especially in the case of puma, bobcat and bear, which were also a problem in the area.
More than how many dogs you have, I'd say that age, breed inclinations (are they hunters or guarders, etc) and personality factors play a big part.
I'd also say that your own personal preparation is important. My husband didn't want me out in the woods without a gun or at least a machete. A compressed air horn is also a great tool both for signalling that you're in danger and for chasing away predators.