From what I could find in the rules,
Letters and numbers are valid clues, as long as they refer to meanings. You can use X: 1 as a clue for RAY. You can use eight: 3 as a clue for BALL, FIGURE, and OCTOPUS
However, they didn't give any example of numbers greater than twenty like "twenty-one", "twenty-two", "four hundred twenty-six","nine hundred eleven", "nine eleven", and "nine one one" which are borderline more than one word, and more than one word is clearly stated in the rules as not allowed unless it falls under the section "Flexible Rules" for pronouns and hyphenated words. Below 21, all numbers are one word like "thirteen", "seventeen" etc.
I'd like to do "911" aka "nine eleven" for the terrorist attack on 9/11 2001. It is probably equivalent in terms of ruling to "nine hundred eleven" and "nine one one", as in should probably be treated the same in the rules (whether allowed or not allowed).