The principal Upanishads are accepted by scholars to be those which Shankarayacharya (CE 788-820) chose to comment upon, they are ten in number and enumerated in the Indian tradition as follows: Isha, Kena, Katha, Prasna, Mundaka, Mandukya, Taittiiya, Aitareya, Chandogya, and Brihadaranyaka.

Scholars are divided as to the date of the composition of the Upanishads. Many of them are agreed, however, that most of the principal Upanishads belong to the period prior to the advent of Buddha in the seventh century before Christ.

Reference: The Message of the Upanishads by Swami Ranganathananda (p.1)