You can have virtually unlimited keywords in your site but it is not possible or impractical to get good ranking for all keywords. And it might look like a spam site and you might get a low or no traffic from se.
I keep it around a dozen per page. You will find if you have optimized well and get some great backlinks you be ranking for many longtail you have never even considered.
Yes, Google does not read the keywords, but for now! But many other search engines are need keywords. Google may also receive further attention to them next time. And I think that Google storage them in the database. If your keywords match your title, description and contents its good and Google use them. Its my idea...
Well, we can't say the exact number of keywords for one page because it totally depends on your content that how much the quantity of the text so that we can adjust our keyword according to that text , we have to look at the keyword density not keyword stuffing while placing the keywords at the content of the site...