Since all of the "number of things" words all follow the same pattern (number plus つ), you really just have to remember the part before the つ (and then remember that the つ is in all of these). All of the readings are the kun'yomi reading, which means we'll have to use a mnemonic to remember them (you know the on'yomi reading of the kanji portion).
So what eight things do you have? You have eight yachts (やっつ). Go ahead and count them. They're beautiful. Also, be sure not to be confused by this particular reading mnemonic. It includes the つ, even though the つ is outside the reading you need to learn (just makes things easier for you overall). Since you can see the つ outside, it shouldn't be too difficult for you.
There are eight potatoes in a bag.
There are only eight of them.
What has eight faces and six arms?