In your example, the S&P is $50 per point and is trading at 1000... the contract is worth $50,000... so that is your total risk. If you buy 2 contracts at 1000 in your $20,000 account, your total risk is $100,000 so you are leveraged 5 to 1.
5yr
Edit:
If the initial margin is $4000, and the maintenance is $2000, the you need at least $8000 in your account to buy 2 contracts. Once you have the 2 contracts you need to maintain at least $4000 in your account otherwise you will get a margin call. Once you get a margin call you will need to meet the initial margin requirements again, you will have to deposit more money or close your position.
5yr
Edit:
If the initial margin is $4000, and the maintenance is $2000, the you need at least $8000 in your account to buy 2 contracts. Once you have the 2 contracts you need to maintain at least $4000 in your account otherwise you will get a margin call. Once you get a margin call you will need to meet the initial margin requirements again, you will have to deposit more money or close your position.
