I once ordered 6 straight run and got 7 pullets. Even the packing peanut was a pullet. It's just the odds. I was in the less than 1% group. Since, like you, I wanted both roosters and pullets, I did not rush out to buy a lottery ticket. I can run the odds of 6 or 7, but how do you calculate the odds of 7 out of 6? 1 out of 64? 1 out of 128? I think I'll get another cup of coffee.
One hatch I had from green eggs gave me one rooster and four pullets. I wanted more green egg layers so I was really happy. But of the four pullets, only one laid green eggs. Promising odds proved pretty disappointing.
I don't know if you are getting them shipped or if you will pick them up. That might make a difference since more to keep each other warm gives you better survival rates for shipping. Even if you pick them up instead of ship, there is no guarantee all will survive or be of the quality you want. If you are getting them from a breeder who has won prize ribbons with them, I can see that an extra that you really did not want could be an expensive meal.
There is a lot about your situation and goals I don't know. With what little I do know, I'd be tempted to go with a few more rather than less. I personally would not go less than 10. The odds of you getting at least 2 roosters or 2 hens is really in your favor with 10, you probably will have a few extra to choose your keepers from, and if you wind up with a real shortage of hens, you can set up pairs instead of trios. If you have extra hens, maybe quads instead of trios. But it is a personal decision. You know your situation and goals a lot better than I do.
