There are eight potatoes in a bag.
There are only eight of them.
What has eight faces and six arms?
This word follows the "number of things" pattern where there's a kanji for a number plus つ on the end. Whenever you see this, you know the word means "____ things." Knowing that, as long as you know the kanji (which you do) you can figure out what number of things it is. For this one, it's the kanji for eight plus つ. So, this one is eight things.
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.