Where would my futures contracts go if the counterparty gets liquidated?

The actual counterparty to an exchange-traded futures contract is a clearing firm. The clearing firm acts as a intermediary between buyers and sellers in a futures market. This is so that traders do not have to worry about counterparty risk (i.e. you don't have to worry about the bankruptcy of the other party). Clearing firms have an "insurance" fund to cover losses from counterparties. Sometimes, brokers are members of the clearing firm and have to financially contribute to a central fund, therefore they also have an incentive to ensure that their clients are actually able to fulfill their end of a futures contract.

What would happen to my futures contracts such as S&P500 futures if the counterparty of my contracts gets liquidated due to a heavy use of leverage?

Nothing would happen. The actual counterparty is a clearing firm. The clearing firm would use its own funds to cover the other end of the futures contract.