With some careful coding, a simple component at its most basic could be assembled with only two files, an XML manifest and a PHP file. In practical use however, the vast majority of components will have at least three files. If they are written according to the best practices for Joomla! Extension development, they will be more likely to have at least 5-10 files. A very complex component, such as a shopping cart or forum, can easily have 50 or more files.
With so many files floating around, how do we know what is what?
This really depends on how the extension was developed. A well-written extension should be organized according to best practices, with similar files grouped together in separate folders. Then it should be relatively straightforward to understand what the files do.