There are more than 100 prepositions in English. Yet this is a very small number when you think of the thousands of other words (nouns, verbs etc). Prepositions are important words. We use individual prepositions more frequently than other individual words. In fact, the prepositions of, to and in are among the ten most frequent words in English.
EnglishClub.com is a great place to start learning about prepositions in English.
Below are some pages with lists of prepositions and example sentences.