Witr means odd number.
The Sunnah is to offer witr in 3, 5, 7 or 9 rak’ah.
One rak’ah alone does the job. However, the best is to pray it as 3 rak’ah as the Prophet salla Allahu alaihi wa sallam used to do.
You can do this either by praying two rak’ah and then offering salam and after that praying one rak’ah or you can pray 3 rak’ah without sitting in the second rak’ah and then giving salam after the third rak’ah.