When a file of the same name is uploaded, it will replace that file with a new 'version.' The version number is denoted with a 'v' under the title of the document. You can view older versions as well as revert the current file to an older version.
Box guarantees to retain up to 100 versions of a single file. After 100, the early versions may begin to be deleted (i.e. version 101 deletes version 1, etc.).