Nine
numeral
When a vocab is made up of a single kanji with no okurigana (hiragana attached to the kanji), it often has the same meaning as the kanji it's based off of. Same goes for the word 九, as well. It has the same meaning as its kanji counterpart.
When a vocab is a single kanji and all alone like this, it's usually going to use the kun'yomi. 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.
九インチ
nine inches
九ドル
nine dollars
九セット
nine sets
ガソリンスタンドまで九マイルです。
It is nine miles to the gas station.
九ページをひらいてください。
Please open to page 9.
このビルには九フロアあります。
There are nine floors in this building.