Since numbers are an exception, this word is read using the on'yomi reading even though it's made from a single kanji all alone. Luckily for you, you learned the reading for this word when you learned the parent kanji.
Tom bought five onigiri at the convenience store.
My friend was born on May 5th in the 55th year of Showa.
Once upon a time, there was a mother pig with five baby pigs.