The answer to this depends on which text you use. If we read the oldest MSS and the texts based on them such as: NA4, NA28, UBS5, W&H, Byzantine text, NIV GNT, THGNT, Souter, etc, we get:
ἑξακόσιοι ἑξήκοντα ἕξ
This is pronounced: hexakosioi hexekonta hex (= six hundred sixty six).
A few hundred years after the first century, scribes started using letters as numbers, a practice called "Gematria". This enabled the scribes to abbreviate the writing of numbers. This is reflected in texts that follow those created in the middle ages such as the Textus Receptus, Stephanus text, the Majority Text, the text of the Orthodox Church, Pickering's F35 text, etc. All these later texts have:
This three letters contain no vowels as it is just (transliterated) "chxs" and is unpronounceable.