It really depends on the size of the kettle, doesn't it? I'd bet that the kettle is somewhere around 32QT, which would mean 8 gallons - at that's the standard size I see at Wal-Mart in their turkey fryer kits.
In an 8 gallon container, you can comfortably fit just shy of 20lbs of grain at 1.25qt/gal. Of course, you'll have some dead space at the bottom where your false bottom sits, but that's still a lot of grain in the kettle. The biggest challenge will be keeping it warm, as mentioned above. You have the option to apply low heat to the base of the kettle and, with regular stirring, you can keep it from scorching. But then you gotta wonder how you're gonna heat your sparge water, if you're using the burner to keep the mash temperature steady.
At that point, it's probably better to use the aluminum kettle as a Hot Liquor pot, into which you've attached a ball valve draining to a sparge arm. Set the sparge arm in the top of a converted 10-gallon Igloo mash tun, and everybody wins.