Here are some examples:
* "anti-" (against) - one syllable
* "inter-" (between) - one syllable
* "super-" (above, beyond) - one syllable
* "un-" (not) - one syllable
But...
* "ante-" (before) - two syllables
* "counter-" (opposite) - two syllables
* "over-" (too much) - two syllables
* "multi-" (many) - two syllables
* "subter-" (under) - two syllables
So, while many prefixes are one syllable, there are also plenty with two or even more syllables.