請問期指係咪無限續期???

Yes. you need to have enough money in your account.

Once your contract is expired, you need to renew it by performing two transactions;
One buy in recent month and one sell in next month contract OR
One sell in recent month and one buy in next month contract.

In fact, by doing the above, you need to pay 2 transaction fees.
If you have two contracts, then the fees is X 2 ....