Every language has the same number of cases, including English. However, in English's case, many cases are implied (or understood by the listener), sometimes with the help of prepositions, and there is no declension or other alteration of the original word. However, there are a few cases of declension, such as for the possessive case ("I vs. my"). This is in stark contrast to languages like Latin and Finnish, where words are declined and altered routinely. I've studied linguistics for a while (it's a small passion of mine), and English has probably the least amount of "word-alteration" that I have seen of the languages I have studied. This can make learning other languages tiresome and difficult for native English speakers. We only really have to memorize how to make words plural and how to make possessives and all of the irregulars. Compare it to Finnish (and other highly agglutinative languages), where there are at least 16 ways to alter a word for certain cases (not to mention irregulars)!