My commercial yeast says to use, on the package, 20g per hl (100 liters) of must. 100 liters are 22 gallons. So, that is about 1 g per gallon. So mainshipfred is accurate, according to my yeast manufacturer.
And of course, white wine and red wine have different first must to total final wine production ratios. Adjust as needed.
That being said, even one active cell of yeast will eventually populate any amount of must, the amount added is really about getting fermentation going quickly to out compete other critters in the must, and to get a fermented product sooner.