We can’t say exact number of fall in banknifty points but by LTP of option you are holding.
Assuming only one option you have shorted, it require around 30k to short 1 lot in MIS, one can lose 50% of cash before RMS start squaring off, so 15k loss you can make, if you have shorted at 100, ltp of it can goto 800 before rms start squaring off. In NRML LTP can go upto 2000 to initiate square off before 3, after 3 one should have full margin to carry it for next day. Also there is no guarantee your position is squared off in flash crash at exact mentioned levels, it can anytime after crossing the minimum or square off can even result in debit to your account in flash crash events. So, it is upto user to manage their own accounts.
Either both will be squared off or both will be not touched, not required to enter them as a spread. Ideally both won’t be touched in normal cases.