A 20 qt. pot should fit fine on a stove top, that's what I use for my mashing kettle when I'm doing stove top BIAB. You can mash over 9 lbs. of grain in a pot that size, but you'll probably want something larger for the kettle that you actually boil in.
With two pots and good timing I can get over 6 gallons boiling on my gas stove no problem. Start heating my mash kettle back up while I'm sparging in my boil kettle, then as soon as I'm done sparging put the heat back on the boil kettle. Carefully pour the mash into the sparge/boil kettle after the grains have been removed, but the lid on, and before I know it I have a boil.