hal's got the numbers correct.
Essentially you lose .5 quarts of liquid into every pound of grain. If you mash at 1.5qts/lb, your available wort without any new additions is 1qt/lb.
There seems to be very little practical difference in the effect of mash thickness unless you're going for a very high OG beer. What if your grain bill is 18 pounds and you mash at 2qts/lb? Your available wort without any sparge at all is 6.75 gallons. Basically it's a no sparge for a 5 gallon batch. For that reason, I mash high gravity beers quite thick so that there's room for at least a bit of sparging.
On the other end of the spectrum, take hal's example. 8lbs of malt, 1.5 qts/lb, that's 3 gallons of strike. First runnings would be 2 gallons. That leaves at least 4.5 gallons of sparge you have to add. If you want to maximize batch sparge efficiency, you would either sparge in two steps of about 2.25 gal each OR add a gallon as a "mash out" addition prior to first runnings OR adjust your mash thickness so that the first runnings is closer to 3.5 gallons. All of those efforts are to get the runnings volumes close to equal.
