at 1.25 qt/lb my 5 gal round cooler maxed out at 12 lbs of grain.
I upgraded to a 10 gal cooler, and haven't maxed it out yet, but I would venture at same thickness, probably around 24 - 25 lbs
i've pushed it and done 30 lbs. (not sure what the water to grain ratio is there). i only got about 65% efficiency though, and it was full to the brim which made sparging awkward.
26 lbs. is about as high as i'd go now.
i just doughed in a 10 g batch of spiced pumpkin ale in my 10 g igloo cooler with a false bottom. there's 26 lbs of grain and pumpkin, 6.75 g of water (1:1 ratio), and a little room to spare.