The correct spelling is "ninety". Although both spellings are used in spoken language, in written language, "ninety" is the preferred spelling. This is because the number is written as "90" which contains two separate letters that are joined together.
It helps to remember the spelling of this number if you can create an association between it and other numbers that follow the same rule. For example, when written as a numeral, "ninety" is a combination of "9" and "0", while "three hundred" is a combination of "3" and "0".
The correct spelling of this number is also the same in all English varieties, including British, Australian, and American English.
If you need more information about the correct spelling of this number, you can find it here: https://www.dictionary.com/e/spelling/ninety/.