The number of yeast cells required at the start depends on volume, gravity, and oxygenation. A pitch that is lower in cell count than it should be can yield the correct attenuation (and thus the correct FG) but be stressed while doing it. If that happens, off flavors happen. Brewing software such as Beersmith can tell you what cell count you need for a given batch, presuming adequate oxygenation. Liquid yeast packs are frequently insufficient for 5 gallon batches; they are intended to be used to make a starter. Dry yeasts have a much higher initial cell count and can generally do the trick for 5 gallons unless the gravity is high. The best thing to do if more cells are needed with dry yeast is to pitch two dry packs.