Google has said in their Webmasters site and via Matt Cutts that meta keywords are not used by them in their ranking algorithm. There is some thought that Yahoo may still utilise meta keywords but the impact is unclear. Given the propensity for misuse of this tag I suspect Yahoo does not place a very high value on it.
I still tend to include them but I only use words that appear in the page content and which support the purpose of the page. I don’t think there is a magic formula to determine what the optimum number is, though I suspect after say 15 or 20 they will be ignored.
Google encourages webmasters to build pages to meet the needs of the website’s visitors rather than the search engines. If you follow this philosophy I suspect the list of keywords will rarely exceed 10 and more generally you are unlikely to breach any of their requirements.