I'm not sure if you are confused or I am confused. Let me explain again then you tell me...
Talking purely about Nifty Futures:
there is Nifty and there is MiniNifty. There is a 1month 2month 3month etc which you should know.
So now in Nifty futures, the minimum you can buy is 50 of it and mininifty 1 lot has 20 of it. Dont ask me what "it" is because I dont know what it is called.
However this should clarify it: 1 lot of Nifty future, if the price is quoted at 4000 then the cost is 4000 * 50 = 2Lakhs but you pay a margin which is about Rs.30,000.
Same for mini nifty if the quote is 4000 then the cost is 4000 * 20 = 80,000. Margin about is about Rs.12,000. Additional amount to compensate for Market to market losses (to avoid margin call)
Basically if you have Rs.12000 you can buy 1 lot mininifty @4000. if the day closes at 3900 then you will have to shell out 4000-3900=100 * 20 (50 for nifty) = 2000.
Hope that clarifies it. So now I guess 1 lot is supposed to be 1 contract?
* All numbers are rough estimates. If you want a demo place your buy sell prices and I'll calculate them that should clear all your doubts.