Bracket order

As in Bracket order we limit our SL, So does this mean that the blocked margin for the position will be as per the SL points or the Full MIS margin will be blocked ?

Margin blocked is based on your stop loss value. That is why is you are getting more than 20 times leverage. Since it is single order, no separate margins for stop loss and target are considered. Only single margin based on stop loss is considered.

MIS margin is not applicable. In MIS you only get around 3 to 9 times leverage. MIS is a different product type

So how much margin I have to maintain to place a Bracket order for one lot of nifty?

https://zerodha.com/margin-calculator/BracketCover/
This is the link where you can check for yourself (for any number of lots) on any scrip or index