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Understanding how often to use a key phrase in a particular page can often give you some insight as to how a search engine ranks web pages. These are called “on the page” criteria. Now, engines like Google really look at “off the page” criteria as well. But that’s a whole other can of worms.
Here is a search page at Google that lists a bunch of places you can learn more about keyword density:
Google Search: keyword density
GRKda is one I’ve used before that is very comprehensive.
But check out the others on that search page. There are a wide variety of them.