I have never tested the validity of this advice, but I have seen it recommended that you not exceed one gallon per pound of grain when steeping. Personally, I have had good success with about 2 quarts per pound for the steep, followed by a sparge with an equal amount. Then top up with water to reach your boil volume (if required).
As Orfy says, you might consider going AG if you already have full boil capacity. A cooler-based mash tun costs very little to make, and assuming you have a chiller and a thermometer already, you probably don't need much else (except a bit more time on your brew day!).