The meta keywords tag is not given much (if any, in some cases) weight by search engines, but since it is easy to populate I think it is still worth doing. Other areas such as title tags, meta description, headings and body text are much more important for SEO.
Put 10-20 words into meta keywords. Separate words/phrases with commas. Make sure each word or phrase you use is mentioned in the page body (otherwise don't bother including it).
Keywords are infrequently used nowadays because so many people used to stuff keywords back in the late 90s and early 00s. But in general I agree with you guys - 20 to 30 on topic keyphrases. Funnily enough, I've seen keywords in meta tags separated by commas and also just by spaces.
Better target 2 keywords per page and target the same keywords in meta tag as well. Keyword prominence and proximity is importance, and your meta title and descriptions need to with keywords at the same time with user readable sentence.
I wouldn't go for too many, 5-10 since you don't want to get accidentally penalized. I read Google doesn't look at meta tags but other search engines might. Everything in moderation shouldn't be of harm.