There's no real limit, since you can just keep dumping stuff in forever. There is however a soft limit that the stuff has to be close enough to get dumped in, and this one is extremely old so we aren't sure where it got its mass from!
Au contraire! There's plenty of mass to feed such a black hole, the real question is "when you run things backward" in the lambda-CDM model why isn't the entire universe a black hole? Or why wouldn't it be the case that there wasn't a primordial mega star that quickly (in universal time) collapsed into a supermassive black hole and the remnants of the supernova is the mass of the remainder of the universe and form the CMB radiation?
This is a cool idea I've never heard before. The universe we're in is the result of a cosmically massive star going supernova.