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a two-word keyword (cheap widgets) will bring in less traffic, but that traffic will be looking specifically for what you offer and is more likely to purchase.
same goes for a three-word keyword (blue cheap widgets) bringing in even less traffic, but even more qualified leads looking for exactly what you offer.
and so on and on with 4-words, 5-words, etc. Again, generally speaking, the more words you have, the easier it is to rank well, the less traffic you receive from the search, but the more qualified leads you receive, resulting in better sales.
I usually avoid one-word keywords, and try for the two-word keywords, with three-word keywords being next best. i dont bother with more than 3 generally.
What is better: to have 10 3-word keywords or better to have 20 3-word keywords on my page?
neither. 10 keyword phrases on one page is too many, much less 20. stick to 1-3 keyword phrases (of any length) per page. if you have that many phrases to play around with, its better to create separate pages for them (which also gives you the advantage of having more content).