Futures have sellers and buyers. Why is there only one contract? Don't buyers and sellers want to sell the contract when they close their positions? See where my description corrected the mistake.
Of course, there is only one sales contract. There are both parties in a contract. If A's position is closed, such as being followed up by C, then the contract becomes CB. If b is also closed, for example, followed by d, then this contract becomes a contract between CD parties. This contract is on the exchange, not on either side.