The main thing is to understand that each page on your site should have a "theme." That`s the single "spin" on your product you want to highlight on that page. For example, say you`re selling candles.
One slant or theme on those candles is that they decorate the place. So "decoration" becomes part of a key phrase. Another spin is that they`re scented. "Scented candles" becomes another theme, on another page.
The idea is to capture as many versions of what someone might search for. Those are your different key phrases.
Now consider how many people search for "candle" but type in "candel?" Do you want them to miss your site because they can`t spell? True, Google may offer a warning, "Did you mean candle?" and someone might see it. Then again, they might not.
Now suppose you`re selling a "bed and breakfast." Or suppose you have something "hand made."
Do you write your SEO content only for "bed and breakfast?" What about "B&B?" You can definitely lose the dashes, so "bed-and-breakfast" becomes the same as without the dashes. But what about "bed & breakfast?" How about "B and B?"
And consider "hand made" versus "handmade." Both words are essentially acceptable to a search engine, but they`re very different in terms of results. Suppose someone makes a typo and looks for "hand maid" candles? That`s legitimate too, but you won`t be picked up unless you have those misspelled words on your site.
Now let`s think about "hand made" versus "made to order," "customized," "custom" (a different word), and "personalized."
The important thing about SEO writing isn`t that you have key phrases in your content, some number of times. It`s about matching on your pages, the *exact* words people are using when they type in a search query.