This word is made up of a single kanji all alone, but it uses the on'yomi reading. This is probably because you'll usually see this word attached to other things, like numbers. This shows the edition or number of an item. So, even though it's alone, it's usually used like jukugo, which is why the reading for this one is the same as the reading you learned when you learned the kanji.
Which train car are you going to ride in?
The first fortune-telling robot with 100% accuracy was made by a genius scientist and a genius engineer.
In Japan, we usually hand out an extra issue of the newspaper for free.