Two. "Ih" and "zent." Well, technically isn't is one syllable, having only one vowel. But the semi-vocalic n sound amounts to a second syllable for the purposes of versification.
But actually, it's two syllables and in no case would it be one. It may be spelled with only one vowel, but with English spelling the way it is that doesn't mean anything. Isn't is pronounced with two syllables. The OED phonetic spelling is /izent/, which contains two vowels. Even if you were to eliminate the e in favor of a syllabic n, or "semi-vocalic", it would still be a syllable, wouldn't it? \
I stand corrected: the vocalic n sound constitutes a syllable all by itself. However, the "phonetic" spelling [izent] cannot be right: no one pronounces the second syllable of isn't with the same vowel they use for men and Zen. We might use the schwa vowel for emphasis, saying something like iz-zunt, but normally we say izznnt, never izzent.
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