When the Open Interest of a futures contract crosses 95% of the Market-Wide Position Limit (MWPL) set by NSE, the stock enters into an F&O ban.
MWPL is the maximum open interest that is allowed for a stock's futures contract. In numerical terms, MWPL is the lower of either one of these metrics:
30x average shares traded daily (in the month before)
20% of the total number of shares non-promoters hold (using the free float method)