The number 90 is spelled "ninety." It is formed from its base of nine followed by a -ty which signifies that the number is followed by a tens placeholder.
The number 90 is often misspelled because many people drop the "e" and leave the word as "ninty."
Numbers between 20 and 100 that are multiples of 10 are spelled from a form of the base number followed by the -ty: twenty, thirty, forty, fifty, sixty, seventy, eighty and ninety. For the numbers between them, simply append the spelled number in the ones place to the spelled number in the tens place. For example, to spell 47, simply add "seven" to the end of the spelled number forty to get "forty-seven."