How many keywords should there in the content page?

You can choose your 3-4 primary keywords in content.Content should be relevant to keywords and unique,genuine...

4-5 keywords should be enough and genuine.

you can add 3-4 keywords in title and promote over search engines.

Use only 3 or 4 keywords in an 400 words content.

I am agree with @josshray but means not you only use 3-4 keywords but important think is keyword density....
keep you keyword density minimum 3-4% only...

I suggest  you to target only one prime keywords per page. With this practice you can sure get good revert from your content.

The better content page contains 3 to 5 percentage of keyword density, More than 5% of keyword density spoils the contents quality.

It should not cross 2 - 5 % of the total content, and if you are targeting too many keywords then in that case focus on only 3 - 4 keywords for the content pages.

This is called ad keyword density. The keyword density of a website should be 3%. You can repeat the keyword 2 – 3 times within the content.

your keyword density should be 3 to 5% not more than that.

According to the new Google policies, you have to maintain the 2~3 % keyword density in webpage content.

if you have a Web page with exactly 100 words on it including all headlines, captions, alt text, and advertising, and you have a keyword phrase that is on the page 3 times, your keyword density is 3%.

Actually I am not getting your question. Are you asking about website content or content posted in other website. If content in your webpages, then there should be only 4-5 keywords are enough.

I think in 100 words 3-4 times is enough.

A web page will benefit if the keyword density of the page falls within the 2 – 3 % range. 

6-8 keywords in a 300 words in a web content are sufficient to have and do not look as stuffed.

In practice, using a large number of keywords is not practical and could actually end up harming the website's search engine optimization (SEO). The words you choose need to be included in your website's content - and your content is ultimately intended for your customers.

It depends upon how many words you have in your content, generally 2 to 4% density should be maintained. So it you have 100 words you can use keyword maximum 4 times in 100 words. Preferred 2% is the best.

The 3-5 keywords are used in 300 words content. It it best ratio for your better seo.