Yes 10-12 at the usual 1.25 quarts. I've done up to 14 pounds with using a lower water to grist ratio. Green Bay Rackers Mash Calculator.
Scroll down to the "can I mash it" calculator. It's a valuable tool to use to figure out how much grain can be hel in any sized mash tun.
I've been using my 5 gallon for 6-7 years and have never had a problem with 99% of my batches. The others, the rare barleywine I either borrow someone's larger cooler, or do a 2.5 gallon batch.