errr.... I wouldn't count on any deaths either. If you order 12 of your choice of a breed, some (most) hatcheries will add 'packing peanuts' meaning extra cockerel chicks to make up the difference so there's enough in the box to give them added stability and body warmth. I'd find someone else locally who wants some too order all the breed you want, then either give them the extras or share the cost. (depends on their and your financial situation, but that's just me. I've shared with a friend just because they were friends, the cost of chicks isn't usually too extreme and I'd rather help a friend)
As far as the accuracy of sexing chicks, most hatcheries say their accuracy rate is 90 to 95% . Meaning in an order of 20 if you get 2 cockerels, you can't complain, but I've never gotten ANY cockerels in my all pullet orders... I know others who have though. I have only ordered from either Meyer Hatchery or Mt. Healthy, both Ohio companies. If you got more than whatever percent they say, you can call and complain and most will credit your account... even if you get any though, I'd call and let them know so they know how their doing, that's just me though.
Good advice about your location and amount of space per bird, that's exactly what I tell people too. Bad weather, more time in coop, plan on the 4 sq foot per bird. Warmer weather less time in coop, safe to go with 2 (or 3). Being closed up with less space means fights and bickering, (think kids locked up in the house due to bad weather, kids holding sharp pointy beak shaped toys that they smack each other with till they bleed?)
//eta// Oh, and the # sq ft / bird is also got to do with the breed. Buff Orps are a large bird, so consider that. They're also supposed to be fairly calm and friendly though so less fighting -maybe- A nervous breed may need more room, but a small breed, less. Yeah yeah, I know, lots to consider, not sure I helped much, but hope I did at least a little.