As mentionned, the Mouth of Sauron was his 2IC, this is implied in ROTK during the parley at the Morannon, when it is mentionned that Sauron will give Isengard to the Mouth out of nepotism. As for the Witch King, I believe he was the chief of the Nazgul but i guess in Mordor's chain of command he was 3IC.
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Ash Nazg Durbatuluk
Ash Nazg Gimbatul
Ash Nazg Thrakatuluk Arg Burzum-Ishi Krimpatul
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