How many keywords are ideal for the META keywords tag?

The answers to a previous question pointed out that the keywords meta tag in HTML is used by some search engines, such as Yahoo! and Ask.

Now, I know not many of us have direct knowledge of the algorithms behind different search engines, but in your experience, does the number and order of keywords have any effect? Is it better to have fewer or more keywords, and are the keywords that are listed first given higher priority?