150 characters are good for meta keywords i.e 10-15 keywords without repeating. But nowadays, Google give less importance to meta tags
Include 10-30 keywords. Even tough meta tags are not as important anymore, sometimes for niche keywords, they still may help.
The meta description tag will display on google search results. Make it descriptive, not keyword rich.
A good descritption will be important to draw in traffic but hopefully your site and keywords will be relevan to each other and It will allow you to fill the meta with a decent number of them.
I try to keep keywords under ten, meta description under 150, and title right at 60. It is true that Google pays little attention to keyword tags, but some other search engines still do...