I use about a cup and a half or two cups to rise two pounds (10 cups) of flour in six hours or so. My starters are generally kept pretty thin, and get used twice a week (or at least fed if not used to bake with). Brewers, who use a different strain of S. cerevisiae, claim that 1 million cells per liter of fermentable wort is the optimum for healthy yeast growth.
Not that that really matters to your pizza dough. My (admittedly empirical) tests indicate that I can use a little or a lot of starter, and that as long as there is plenty of fermentables (food, i.e. flour), the dough will be living when I need it, and I can't really tell much difference in taste or texture, if any at all.
But, since we all know that rigorous testing is fun and delicious, I think I'll head to the lab this weekend.