Since this word is made up of a single kanji, it should use the kun'yomi reading. When learning the kanji, you didn't learn that reading, so here's a mnemonic to help you with this word:
Imagine yourself counting by playing a kazoo (かず). Each number you count, you toot your kazoo. How fun!
Oh weird, the numbers don't match for some reason.
What was the death toll of the 1985 New York earthquake?
In this movie, the role of the math teacher who is good at counting is played by Koichi, who is known for playing the role of numerous older brother figures in countless Japanese films.