90 can be spelled in different ways, depending on what context it is being used in.
If you were writing out the numeral, you would say "ninety" or sometimes "90."
If you were using it to represent money, you would say, "ninety dollars," or "ninety cents."
If you were writing it as a part of a larger number, such as in a phone number, you might say "nine-zero."
If you were saying it out loud, you could also say "nine oh" instead of just "ninety."
For more information, check out this page from Oxford Dictionaries: https://en.oxforddictionaries.com/spelling/numbers
