The plural of syllabus is syllabi or syllabuses.
A plan or summary of subjects to be taught on a particular course of study.
The word syllabus comes from Latin and words derived from Latin with the suffix -us replace this with -i to form their plural.
The Latin pluralization of syllabus is therefore syllabi
The standard English rule to form the plural of a word ending in -us is to simply add the suffix -es. According to this rules the plural of syllabus is syllabuses.
This is one of a small number of examples where an English word has two acceptable forms of plurals. You can use either syllabi or syllabuses and both are quite common in the English language.
As seen in the graph below, both plural forms of syllabus have been used interchangeably in written English since 1900 although the word syllabi is more commonly used these days.
Singular Example: The class groaned as the algebra syllabus was revealed to include polynomials and their graphs.
Plural Example: The deadline for the publishing of this year's syllabi was fast approaching
Plural Example: The syllabuses combined included a huge wealth of knowledge and skills to be learned in such a short space of time.
The graph shows the occurances of the plural of syllabus in written English since 1800 using Google's Ngram Viewer.
Engineering and Medicine are believed to be some of the toughest courses in the world, owing in part to the intensity of their syllabuses.