I have one mountain bike with its own tyre. I would like to turn that to a hybrid. My MTB tyre size is 26×2.125 57-557 what size should I purchase if I am going with hybrid tyres?
You obviously need a tire of the correct diameter for a "26 inch" wheel - These may be marked 26" or ISO / ETRTO 559.
Generally you'll want a tire narrower than a MTB tire for on-road use. Narrower tires running a higher pressure have less rolling resistance.
How much narrower is up to you depending on the roads or trails you want to ride on, but you are limited by the width of the rim. See What is the maximum or minimum tire width I can fit on my bicycle.