1, 2, 3, IV, V, VI etc. are unacceptable, for obvious reasons. By extension, some major trilogies are excluded because one or more films had no subtitle, or only had numbers.
However, non-integers and numbers greater than 7 will be accepted, for what will become obvious reasons. Lastly, PLEASE let me know if this has been done before. I could have sworn it was, but I can't find one.