User blog comment:Grizzly Bear/Numbers- Words, Not Numbers/@comment-10950063-20141203181706

AP style is that numbers under 10 get written out. Numbers over 10 are done with numbers. If it's at the beginning of a sentence, you spell it out no matter how big the number.

Ordinals are the same in AP style. First through ninth and from there it's 10th to whatever.

Different guides will say different things, like that numbers under 100 can be written out. It's somewhat of a preference thing.