If you are ordering the keets from a hatchery, get their minimum shipping requirement. If you end up with more than you really want, they are usually easy to sell since someone is always looking to replace their lost guineas.
If buying locally, I never recommend keeping a flock of fewer than 10 so starting with around 15 would probably get you there. The larger the flock, the more they use their methods on themselves instead of picking on other poultry.
