"There is nothing new under the sun but there are lots of old things we don't know." ~ Ambrose Bierce, American journalist (1842–1914)Straight from the lips of Google... from Susan Moskwa of Google Webmaster Help:
Q: How many keywords should I put in the meta keywords tag? I heard the limit was 25, but then my web guy told me you should only put 3 - 5.
A: Not all search engines take the meta keywords tag seriously these days, since its potential for abuse is so great. If you're using it because you think it's a great way to send Google a signal about your site's topic, your time would probably be better-spent creating great content rather than carefully tailoring your meta keywords (since site content is one of the signals we use to determine what a site is about).So Google says time is best spent creating great content... creating value for your readers creates value for Google.
Filling the meta keywords tag with hundreds of keywords tends to look like spam, so I'd keep the numbers reasonable; but if you want to use keywords, there's no exact limit on how many you're "allowed". I've heard several people say they use the meta keywords tag as a reference for themselves, so that when they come back to a page they can remember which keywords they were targeting or what the main focus of the page was (this is particularly helpful if you're managing a large site with many pages). Using the number and type of keywords that would be useful for *you* is a pretty good rule of thumb for the meta keywords tag.
I'm still using my Keywords Tag, (Description tag as well), considering what Google says publicly and what Google actually does is not always the same.
Yes I'm a cynic, but again, inserting these tags takes no time at all and I'd rather be safe than sorry - until Google says using these tags will penalize you.