The folks on those rigs drilling the relief wells had damned well better be prepared to pump a hell of a lot of mud BEFORE they ever get to the casing as this thing is flowing OUTSIDE the pipe!
Well, I haven’t seen any reference to the well flowing outside of the casing in the wellbore, and would welcome a link.
Either way, it isn’t just the quantity, it is the quality. the mud weight has to be enough to exert enough hydrostatic pressure to kill the well, regardless of whether the well is producing in an annular space or not. I am sure they will have a lot on hand.