Based on our experience, with anywhere from 2 to about 15 in a flock, guineas aren't super effective at reducing ticks. They're also very noisy, leave big turds

Based on our experience, with anywhere from 2 to about 15 in a flock, guineas aren't super effective at reducing ticks. They're also very noisy, leave big turds all over the place, and at least in our rural New England area, serve to attract predators. Our spring "dog tick season" has become intense the last couple of years. At the height of the season this spring the daily tick count from grooming the dog was anywhere from 10 (a light day) to a high of 27 ticks acquired after an hour-long hike. When it's this bad, I can pick up four just walking across the front lawn. During that season they're mostly not deer ticks, so fortunately they're bigger and much easier to find, and at least according to most literature I've seen, dog ticks do not transmit Lyme.