One phenomenon of Fast Speech Rules in English
(besides centralization of unstressed vowels to shwa),
is reduction of consonant clusters. This is especially clear
when the ordinal number "-th" /-θ/ suffix is added to a cardinal number
that already ends in a consonant or -- especially -- a cluster of consonants.
These are 6 /sɪks/ and 12 /twɛlv/,
which are officially sposta turn into
the ordinals /sɪksθ/ and /twɛlfθ/
(with automatic devoicing of /v/ to /f/, preceding voiceless /θ/
but this devoicing often goes to completion, deleting the /f/ altogether),
so frequently they get elided to /sɪks:/ or /twɛlθ/,
especially if they're contracted with a pronoun, which is very frequent with ordinals:
- /ðə'sɪkswənɪz'mayn/ 'The sixth one is mine'
- /ðə'twɛlθwənɪz'mayn/ 'The twelfth one is mine'
This is compounded, and even more complex, when adding the noun plural "-s"
suffix to an ordinal number, which is one way we're sposta mark rational numbers:
- /wən'sɪksθ/ '¹/₆; one-sixth' /wən'twɛlfθ/ '¹/₁₂; one-twelfth'
- /fayv'sɪksθs/ '⁵/₆; five-sixths' /fayv'twɛlfθs/ '⁵/₁₂; five-twelfths'
That's officially. But in fact clusters like /ksθs/ and /lfθs/ are way too many
consonants in a row for an English speaker to pronounce
at the end of a word, at least at normal speaking rates,
unless one is enunciating very carefully.
So we tend to shorten ordinals in practice, almost always by deleting the /θ/,
leaving /fayv'twɛlfs/ and /fayv'sɪkss/ (with a definite long [s:] in "sixths").