The maximum subtest score on the WISC-V is 19, just like on the other members of the Wechsler family, including the standard norms for the WISC-IV and WPPSI-IV. We are all still patiently (or not so patiently!) awaiting the release of the extended norms, which would allow for higher subtest and index scores to be derived. They were projected to be out in early 2016, but aren't.

...pronounced like the long vowel and first letter of the alphabet...