Welcome to BYC. Glad you decided to our flock. I don't know the answer to your question about how many chickens you can have in Yuma, but I can help you with your question about the best layers. The best layers in the chicken world are White Leghorns (for white eggs), and Black and Red Sex Links (for brown eggs). All three of these are egg laying machines, churning out more than 300 eggs per hen per year, but I personally prefer the sex links as White Leghorns are high strung and flighty. My Leghorns screamed bloody murder whenever I handled them (it's why I don't have Leghorns anymore). Both sex link varieties are great, but I personally prefer the Black Sex Links over the Red Sex Links as my Blacks have been friendlier, and have laid slightly better in really cold winter weather. But you can't really go wrong with either sex link variety. Please feel free to ask any questions you may have. We are here to help in any way we can. Whatever kind of layer you decide to get, good luck with your flock.