The stock index futures contract has no seller, it is designed by the exchange. Investors directly participate, there is no fixed total limit, and they can hold unlimited positions. If you want to buy a contract, you need to find a seller in the market. If the price is suitable for both parties, you can make a deal. The money that investors buy stock index futures contracts is the deposit, which is generally 10% of the contract value. This part of the funds is called deposit to prevent future losses. It's still in your own account, but it's frozen. When the contract comes out, return it to your own account.