According to the MLA (Modern Language Association) Handbook for Writers of Research Papers, you should NOT number the entries on a works cited page. You should alphabetize the entries, and indent the second and succeeding lines of each entry. If the entry starts with a title (when there's no author's name), you should ignore 'a', 'an' and 'the' and alphabetize by the first real word. For example, this should be alphabetized under 'I':
An Inconvenient Truth. Dir. Davis Guggenheim. Perf. Al Gore. Lawrence Bender, 2006.
Unfortunately the MLA Handbook is not available online, but there are several sites that have well-organized information based on the MLA Handbook. I have given you a link to one of these.
It would be worthwhile to differentiate clearly between three different bur related terms before I get down to the question of numbering bibliography. These three terms are references, footnotes and bibliography. All three terms are connected in some way with giving information in one written work about other sources of work on the same subject.
References give details of sources from which material is actually used in writing of the paper. Good and ethical writing practices require that all such matter is suitably cited in the main text of the work. In addition more details of the sources used is given separately to enable reader to locate and refer to these sources in general. There are two ways of doing this - list of references, called 'works cited' in MLA style, and footnotes. Bibliography is a list of source available that provide additional information on subject covered in the work. It is not necessary that all of these sources should have been cited in the main text.
There are different formal standards for citing, giving footnotes, references, and bibliography. Many of these systems (such as MLA style and Harvard Style) do not require list of references or bibliography to be numbered, Instead the lists are arranged alphabetically. There are other referencing styles that require all citations to be numbered in the main text and reference list to be given at the end of the work or at the end of each chapter of the work with serial numbers corresponding to those in the main text.
Another way of giving references is as footnotes. Standard guidelines are available for these also. The footnote have to be numbered serialiy, or maked in some other way so that each footnote can be easily linked to the corresponding part in the main text.
In case of bibliography there is no need to link the entry in bibliography with any citations. Therefore,, it is best to arrange entries in bibliography alphabetically.