This depends on the number of gears on the rear axle of your bike. For bikes with a single sprocket on the rear axle and a single chainring on the crank, you would need a single speed chain. These come in two sizes 3/32 inch (2.38mm or Narrow) or 1/8 inch (3.17mm or Wide), some internally geared hubs can use a chain that is of a different speed, so please check your chain and hub recommended chain.
For rear axles with 5,6,7,8 gears a 6/7/8 speed chain can be used. For 9,10,11 and 12 speed chains you will need to get the corresponding chain for the number of gears you have on the rear axle. It is recommended to buy a chain that matches the brand of your groupset.