Having never done pears, I can't answer the question about whether or not to include the pressings in the fermentation. My gut tells me that you'll get more body and flavor from doing so, but pear skin always tastes sort of bitter to me. Hopefully someone with pear experience will jump in.
As for how many pears to use, I'll share with you my purist fruit wine opinion. If you want a full bodied, flavorful wine, don't use any water, just the natural goodness you get from your fruit. Pear is a pretty lightly flavored fruit compared to some others, making my opinion of adding no water even stronger. If you have access to all you want, I'd suspect that you could get a full 6 gallon carboy or pure pear wine from 100 - 120 lbs of pears.