Unless you are able to buy them locally, most online hatcheries have a minimum of 15 to 25 because of their small size. I have found that keets are really easy to sell around here. I seldom have to advertise to sell keets.
Some people seem to be able to start sexing keets at 4 weeks old but I have not had much success sexing them until they are 6 weeks old. Guineas are not like other fowl in that the ideal number is supposedly 1:1 for the hen to cock ratio. I like to have a couple of more hens than cocks but have seen cases where the cocks greatly out number the hens and it worked out fine.
I try to keep a minimum of at least 10 guineas at anytime. I currently have 13. Mine are housed separately from the chickens and turkeys but do on occasion get to free range together without any issues. Previously I had raised guineas with the chickens and turkeys. At that time none of my roosters or turkeys had any tail feathers due to attacks from the guineas. It is hilarious to watch a guinea latch onto a turkey's tail or wing feather and go for a "sleigh ride" as the turkey tries to get away. It is highly stressful for the chickens and turkeys.