Originally Posted by m1batt1
I'm far from an expert on these things but it seems to me that even at the higher gravities a single packet of yeast would be enough. Considering in every batch that you pitch dry yeast it has to go through some serious reproduction before it can start the active fermentation.
every time a yeast cell splits, it is giving half of itself - its skin (membrane), its guts, etc - away. so yeast can't reproduce indefinitely. eventually they run out of stuff to give away and still make a viable cell. typically a yeast cell can create about 4 new cells before being too weak to split again.
use a pitching rate calculator, like http://www.mrmalty.com/calc/calc.html
, to figure out how much rehydrated yeast to add (it's important to rehydrate!). mr.malty says that 1.3 packets is required for 5.25 gallons of 1.074 wort, so 1 packet isn't ideal but it's probably close enough. if you don't rehydrate, 1 packet probably isn't enough since many (Jamil says up to half) of the cells will die due to sugars and other things rushing inside the cell walls. 0.5 packets instead of 1.3 is a big difference, and that might explain attenuation issues.