I refer, of course, to a matter routinely, if implicitly, raised by the auditors of curricula, every time they ask for samples of a syllabus: if they request more than one, what do they say they want? Syllabi? Or syllabuses?1
A highly scientific anti-prescriptivist study has it that the answer is “syllabi.” I prefer “syllabuses,” though.
If your etymological antennae are twitching, you can find a detailed account of the story of “syllabus” at the specialist links in the first sentence of the post. But the short version is, it’s a made-up word, erroneously thought to be adopted into Latin from the Greek, which it wasn’t. I.e., there isn’t a true proper correct answer, horribile dictu.
1I’ve never actually heard anyone insist it’s syllabūs.