Updating content for higher ranking on google? how much content is too much?

I wonder if its better to just add some new content on old posts, or if its better too make new posts?
when do we speak about too much content on a post? one of mine posts that I updating regulary now has: Total number of words: 5749

Do all the posts need the main keyword in title? or will google marked it as keyword spam?
Il hope someone can give me some answer