When a vocab is made up of a single kanji with no okurigana (hiragana attached to the kanji), it usually uses the kun'yomi reading. Numbers are the exception, as you might already know. When they're alone with no okurigana they're the on'yomi reading, which you should already know from learning the kanji that makes up this word.
I study for about nine hours every day.
Please open to page 9.
I used to have nine mynah birds.