Watching the mission, I noticed that engines kept shutting down. There are 33 of them. When they had turned off 7 of them, with 26 engines remaining lit, it started to lose control. The engineers all seemed ecstatic about the mission. I have to wonder if they weren't intentionally shutting off engines to see how many could fail while still retaining control. If that was indeed tested, the answer appears to be 6--they can operate nominally with 27 engines lit.
It didn't start to lose control because the engines. It tried to do a stage separation flip and the stage didn't separate. Only then was it doing something that might have been out of control, or maybe it was trying to do the seperation again.
Control can be maintained with very few engine. I depends if its center engines or outer ring. Only the central cluster can gamble, 2 or 3 of those should be able to handle to rocket depending how full it is.
With only outer engines the could use differential thrust but the rocket isn't really designed to do that.
The Soviet N1 used differential thrust only. SpaceX prefers gimbled engines.
