According to Designing Great Beers commercial pitching rates are 10 million cells per ml or wort, or about 200 billion cells for a 5 gallon batch. This in a home brew setting means to reach commercial levels you would need an 11 liter (almost 3 gallons) starter for a 5 gallon batch!
Since that is completely unrealistic in a home brew setting, he suggests a good number to shoot for is somewhere around 10-20 billion cells as an absolute minimum.
A dry pack contains about 20 billion cells / gram but only have around a 10-20 percent viability, so 1 7g or 11g packet is lowish in viable yeast count, but sufficient for most needs.
Liquid yeast (Both WhiteLabs and WYeast) according to his counts contains between 1 and 5 billion cells, which isn't enough. He suggests a 1 quart starter should contain about 20 billion cells when finished, but I prefer 2 quarts to 4 quarts for my starters... more yeast is gooder.