Proponents of keyword density formulas will quickly spout out a percentage: 4%, 6%, 12%. However, I wonder where they get these figures from. If you do a search in Google for any keyphrase (say [cast iron frying pan], for example), you'll immediately see why keyword density formulas don't add up. Either version - cast iron or cast-iron - bring the same listings on the search engine results pages (SERPs). Please note: I was looking at the source code for each page so as to include mentions in tags as well as on the page.

Cast Iron Frying Pan

Your results may be different than what I see, as everybody does not view results from the same database. But, when I type in [cast iron frying pan] (no brackets, of course), the first site that comes up is http://whatscookingamerica.net/Information/CastIronPans.htm. Total word count: about 1,611. Keyword density for [cast iron frying pan]: 0%. Keyword density for [cast iron]: 3%. Keyword density for [frying pan]: < 1%.

Next up, Ask Yahoo: http://ask.yahoo.com/20000419.html. Total word count: about 622. Keyword density for [cast iron frying pan]: < 1%. Keyword density for [cast iron]: < 1%. Keyword density for [frying pan]: < 1%.

In the #3 position is Amazon.com at http://www.amazon.com/Lodge-Logic-Pre-Seasoned-Cast-Iron-Skillet/dp/B00063RWUM. Total word count: about 2,405. Keyword density for [cast iron frying pan]: < 1%. Keyword density for [cast iron]: < 1%. Keyword density for [frying pan]: < 1%.

Mexico Cruise Vacation

Here's another example: [Mexico cruise vacation]. At #1 we see Cruise Web, http://www.cruiseweb.com/MEXICO.HTM. Total word count: about 488. Keyword density for [Mexico cruise vacation]: < 1%. Keyword density for [Mexico cruise]: < 1%. Keyword density for [cruise vacation]: < 1%.

eCruises.com is up next with their page found at http://www.ecruises.com/. Total word count: about 238. Keyword density for [Mexico cruise vacation]: < 1%. Keyword density for [Mexico cruise]: < 1%. Keyword density for [cruise vacation]: 0%.

It goes on and on. Yes, you do find some sites that have really high keyword densities, but it is not a given that attaining a certain across-the-board keyword density will guarantee you good success. In fact, from what I've seen, keyword density has not been a valid measure of SEO copywriting success in years. I believe it once was, but Google and other engines quickly plugged the loophole.

Does Frequency Not Count At All?

Does that mean that engines don't give any consideration to how often keyphrases are used within the copy? No. In fact, The Official Google Blog recently did a series entitled Technologies Behind Google Ranking, http://googleblog.blogspot.com/2008/07/technologies-behind-google-ranking.html. In one of the posts, the author states, "The core technology in our ranking system comes from the academic field of Information Retrieval (IR). The IR community has studied search for almost 50 years. It uses statistical signals of word salience, like word frequency, to rank pages." He continues with, "IR gave us a solid foundation, and we have built a tremendous system on top using links, page structure, and many other such innovations."

Other Google documents make mention of the need to include key terms in your copy, so it is established that keywords in copy can play an important role. However, I do not see evidence that a standard, across-the-board "keyword density" is at play. Not to mention, forcing phrases into your page text to the point that it sounds utterly stupid makes no sense. It's not going to help your rankings (except maybe on some sub-engines), and it will almost certainly turn off your site visitors.

So, back to the original question: "How many keyphrases are enough?" That's a judgment call that comes with experience. Here are a few guidelines - not carved-in-stone rules - but guidelines you can consider. And no, don't do them all every time.