When I submit a site to the following meta tag analyzer it suggests to lower keywords to not more than 20 (in order to make the page robot friendly). Yet, I've seen pages that are ranked on the top with more than 20 keywords.
Up to 20 or 25 keywords is probably about right, but the major search engines now pay little or no attention to the keywords. It is much more important to have good, search-engine friendly content which is why there are plenty of high-ranked pages with many (or no) keywords.
"If youíre not using valid HTML, then you havenít created a Web page. You may have created something else, but it isnít a Web page." - Joe Clark
Some search engines don't consider the META tags important or give it very little consideration (Google).
There are still alot of SE that you can optimize your meta tags for (yahoo, msn, altavista etc).
The reason it states you to lower your keywords is because many search engines will not read past the a certain amount of characters/keywords and making the irrevelant. So makes sure you add your most important keywords up front. - Don't repeat a certain keyword / keyword phrase more than 3 times.
If you add more, No harm is done. The goals is to target the main focus points of your website = 20 keywords.