When you do SEO for a website, do you usually need multiple keywords to rank in the search engines?

[Jake]

Some of the major search engines, including Google completely ignore the meta keyword tags, it was a too easy to misrepresent a site there, now the search engines examine all of the page content to determine keywords.
Certainly for most businesses multiple keywords are used, some are considered "money" keywords that attract people ready to buy, others are more indirect, attracting people with a passing interest in your topic. It's been reported that 50% of the searches Google sees are long tail searches (lengthy phrases) Google has not seen before. Just having lots of content on your subject is really the only strategy to match those searches.

Single keyword targeting is used in exact keyword match domain names like DietPills.com, these have given a great ranking advantage in the past but their influence may be diminishing.