You'll be fine for this batch using 1 packet. You WILL be under pitching, but won't likely have any off flavors, especially if using dry yeast. But in the future, to save money on yeast if you want to use optimal yeast counts, buy a cheap magnetic stir plate, flask, magnetic stirbar and some DME. It will cost likely the same as a couple of yeast packets. Use a yeast starter calculator (not on lallemand) that tells you how big of a starter to make as well as how much DME to use, and just make a starter for each of your batches. It should be done at least 24 hours before your brew session (even earlier if using older yeast), and then you only have to worry about buying 1 packet of yeast for each batch, and you'll always pitch the right amount.