Hey there! Melissa here, from the Learning Team.
The number of keywords that goes in each cell correlates to the number of keywords you're optimizing a page for.
You might ask yourself, "Well, how many keywords should I target per page?" Unfortunately, there's really no hard and fast answer here. The number can be as low as 1 or as high as 15 or 20. You need to know which keywords have actual relevance to the page content, and it can get a bit tricky. Remember, focus on topic and concept, because that's what aligns with the Google Hummingbird update. The safe and generally effective bet is to choose one primary keyword that's specific and relevant to a page, with a decent monthly search volume, and then back it up with a few secondary keywords that are also very relevant, but aren't your primary keyword and may not have as high of a search volume.