From White Labs faqs.
Some homebrewers now want to pitch more yeast in 5 gallons then a pint starter. An often quoted number is to pitch 1 million cells/ml/degree Plato of beer, which equals about 250 billion cells for 5 gallons. That is okay, more cells are not detrimental until about 400 billion cells. For those that enjoy yeast culturing and want 250 billion cells, one vial can be added directly to 2 liters of wort starter, and after two days of incubation, will be equal to roughly 250 billion cells. Is this necessary? Every brewer will have a different opinion, but here is some information:
a. The source of the 1 million cells/ml/degree Plato figure: Professional brewery literature.
Most professional breweries re-pitch their yeast because they have the fermentor design and facilities to reuse yeast. So most brewery pitches are actually re-pitches, and only 2-10% of brewery pitchings are using freshly propagated yeast. One of the main sources of contamination in a brewery is the pitching yeast. So in order to out-compete other organisms, large quantities of yeast must be pitched. When propagated by a professional yeast laboratory, the yeast is grown under sterile conditions, sterileoxygen and special nutrients are used to improve cell construction and performance. This does not occur in a brewery, so numbers they use to "pitch" take into account the inadequacy of their brewers yeast. The yeast is also unhealthy due to prolonged growth without oxygen and nutrients. In addition, brewers yeast will always contain some contaminants that need to be out-grown, and 1 million cells per ml per degree Plato has been found to be the best marriage of high pitching rates and no negative flavor effects (Higher pitching rates can lead to unhealthy yeast and a "yeasty" off bite). Liquid yeast grown by a professional laboratory should have no contaminants, so out competing contaminants found in the pitching yeast is not a concern.
One thing that contributes to flavor contribution in beer is yeast growth. If less yeast is pitched into beer, more yeast growth takes place, so more flavor compounds such as esters are produced. Depending on the amount produced, this is how pitching rates can have a direct effect on flavor profile. If 5 to 10 billion cells are pitched into wort, this definitely has a negative flavor impact in terms of higher ester levels and potential for bacterial contamination. But does a pint starter worth of yeast (30-50 billion cells) pitched into beer tasted different then 2 liters worth of yeast (250 billion cells)?