What is the name of the emperor whose Arjun was sarthi in mahabharat?

It appears you have mixed up some names and facts in the question.

In the great battle of Mahabharata, which was fought between Pandavas and Kauravas, Lord Krishna was Sarathi or Chariot driver of Arjuna.

Neither Arjuna nor Lord Krishna qualify to be called emperors. Arjuna was one of the five bothers, who formed the Pandavas side of the two warring groups in the war of Mahabharata. As Arjuna was not the eldest of the brother, he never became an independent king, let alone an emperor. Udhishthir,the Eldest of Pandavas brothers, was a king, but I am not sure if he ever adopted the title of emperor.

Krishna, a friend of Pandavas, and particularly Arjuna, was King of Dwaraka. To my knowledge, he made no attempts ever to expand his kingdom or to adopt the status of an emperor.