Do you mean take with you into the cabin, or check into the hold? Either way, It varies, depending on airline, class of travel and destination. For example, the economy airline, Ryanair, allows one piece with maximum weight of 15kgs checked, and one piece up to 7kgs in the cabin. BA in standard class on transatlantic flights allows 2 pieces up to 32kgs each checked, and up to 12kgs in the cabin.There are also restrictions on dimensions and contents. If in doubt, check with the airline before travelling.