Nine or Ten, in British and American cultures, which number means the maximum?
As stated in the title, it confuses me.
Doesn't "She is a ten." mean "She is a perfect woman"?
Doesn't "She is dressed to the nines." mean "She is perfectly dressed."?
If my understanding of the two sentences above is correct, then, ten or nine, which one means the maximum on earth?

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"Dressed to the nines" doesn't mean dressed "perfectly". It means being dressed up or dressed formally (tuxedo, evening gown), as though going to the opera or a ball.

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Generally, ten represents perfection, but there are some instances of nine having been used throughout Western history. Here’s a relevant article for you:
https://www.phrases.org.uk/meanings/dressed-to-the-nines.html

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Both are slang sayings and in both cases she is looking good!  If someone is dressed to the nines, they are dressed to impressed in very nice clothes. If a person is a 10, they look really really great on a scale where 0 is worst and 10 is best