I was using the Can I Mash It calculator, along with the ratios I get on my equipment, including boil off, loss to trub, loss to grain absorbtion, and loss to contraction during cooldown. Your ratios would be slightly different depending on your pot diameter and your process, but here's the #s assuming my ratios.
A 9 gallon pot will fit around 12.5 lbs of grain in a full volume 5.5 gallon batch, given about 0.5 gallons to spare.
Given the partial volume at 1.25 qts/lb for full conversion, you could mathmatically convert 21 lbs of grain in a 9 gallon pot, which would fill the pot to 8.25 gallons. You would need to dunk sparge or top off water to get back to an 8.5 or so gallon boil, which would be a 6.8 gallon max final yield post-boil.