The volume (trading volume) of a futures contract is usually given cumulatively for a day. It refers to the number of concluded contracts on that day. If there is a market with only two participants (A and B) the volume is zero before a transaction takes place. If party A sells party B a future the volume increases to one. If party B sells party A the Future back the volume on that day increases to two because two contracts (ie bought and sold) have been traded.