How many keywords per content page
A common mistake people make when optimizing their content pages is focusing on a whole list of keywords instead of just one. The best course of action is to focus on one really good keyword per content page and build everything around it. This looks better to the search engines and makes it easier for you to create the content in a way that is very relevant to the keyword or phrase you are trying to capture.
Too many keywords can look spammy, especially if some are not relevant to the rest of the content on the page. Sometimes in SEO it is good to try a multi-keyword approach but in this case less is actually more.