13, maybe 14.Explanation
We go to the Poisson Table and consult the column for r = 20, at the very end of the table. Those values are the associated frequencies. If we multiply each of them (mentally) by 20, we will get the number of pizzas, out of this group of 20, possessing each given degree of pepperoni-pepperedness. The smallest frequency that gives 1 or more probable occurences in 20 trials is that for p(13) = 0.0271, which, times 20, equals 0.542, which in turn just barely rounds up to 1. So nothing sparser than that is very likely to turn up in a run of 20 pizzas.
13 pieces of pepperoni on a pizza is not very wonderful, but at least no slice of that pizza is probably going to be wholly naked of pepperoni. We can say it's presentable. But is presentable good? For an angle on that question, see the next problem.
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