What is the acceptable amount of meta tag keywords for your website that the three major search engines like? More than 200 and less than 500?
The major search engines don't read them. I can't think of any reason to put than 5 or so, just to 'catch' some of the minor seachbots. How on earth can your site be about 500 different keywords? Even if you were rewriting wikipedia, you wouldn't put all the keywords on one page.
worthless they can hurt you in major search engines if you use the wrong words so really whats the point of them?
Although the Meta Tag keyword isn't used by the major search engines I always ensure I add my keywords to the tag for other directories and less popular search engines. Anyways the general rule of thumb is to not include more than 10 keywords however I like to keep between 3-5 as this keeps the percentage per keyword around 16% which is advised.
willyboy104 is right on the nose. The major SE's ignore them, but the small ones don't, so its worth a few minutes to plug em in. Don't overdo it, and don't count on it being the one thing that pushes you to the top.