You're allowed to apply for 6, unless you apply for medicine/dentistry/veternary science, in which case you are allowed up to 4 places. There is a rumour however that from 2008 entry you'll only be allowed 5 places....
You can apply for any 6 courses you want, but you can only have one Personal Statement for all of the courses so it's the norm to apply to 6 related courses (e.g. it'll be impossible to write a good PS for both Physics and English).
You should be able to find out basic information about the application system from your teacher/from books/from researching more on the Internet.