Just one more wouldn't hurt. Heck, nobody hardly notice. Easy peasy...
Hmm.. they would need a friend their same age, so you'd better get him/her a buddy. So get just two.
Two - well, what if you got both boys. That would not be so good. I'm sure you'd like to guarantee that least one boy and one girl from your pair. If you run the math, you'll find that if you get eight it the odds of all eight being the same sex are astronomical. But with just seven, that's not the case. So get eight.
Eight - well shucks, now that you are going to get eight, you might as well take advantage of the "buy 5 get one free deal", so pay for ten, and you'll have twelve.
Twelve - golly, that's not that many really. Might as well bump that to 16 to make it evenly divisable by eight so pay for fifteen, you'll get three for free for a total of eighteen.
Eighteen - that way if one gets sick you still have two sets of eight.
Two -- wait, if you have two sets of eight, why can't you have three sets? Your friends ask, "How many chickens do you have", and you answer, "Three sets".
Go buy twenty four birds.
Everything will be just fine.
Honest.