When does yeast growth stop?

When and why do the yeast cells stop multiplying?

Is it when they run out of oxygen or nutrient?

Or, strangely enough, do they just KNOW when there are enough of them?

The reason for this is to increase the yeast cell yield from my starters.

Is it when they run out of oxygen or nutrient?

Click to expand...

Either condition. People use stirrers to add additional O2.

heres a link to yeast on wiki
http://en.wikipedia.org/wiki/Yeast
A quote explains the o2 is what prevents fermentation from starting.
"In 1857 French microbiologist Louis Pasteur proved in the paper "Mémoire sur la fermentation alcoolique" that alcoholic fermentation was conducted by living yeasts and not by a chemical catalyst.[11][13] Pasteur showed that by bubbling oxygen into the yeast broth, cell growth could be increased, but the fermentation inhibited – an observation later called the Pasteur effect."

When they run out of nutrient or there is sufficient toxic stuff built up to stop them (our old buddy alcohol being one of those things).
Oxygen increases the amount of growth, it doesn't determine whether growth happens at all, at least for yeast.
That said, since yeast need oxygen to make sterols, sterols will become a limiting nutrient w/out oxygen, unless there's added sterols for them. Trub does have sterols in it, so that can help.

There are four things I know of that will make your yeast crap out early:

1) They run out of oxygen
2) They run out of food (sugars in the wort)
3) The alcohol in the beer exceeds their tolerance
4) The temperature drops below their tolerance.