According to that PDF, the OP might get away with 2 bags. I find that PDF hard to believe. Can the little piggies actually eat THAT MUCH????
ERROR, correction: that PDF said 10 birds could be expected to consume 20 - 29# in a 6 week time period. Therefore, I'd not buy more than 1 bag at a time. Do you have any where closer that you can buy feed if you need to? I'd rather buy a less favored brand that was fresh than buy too much of my favorite brand and end up giving them stale feed. This holds true with all birds, but especially for chicks where you have no margin for error.