An article optimized for SEO shoould have the keywords you target in : Page title Article title H1 First sentence in the article and 2 times in the body. I wouldn't recommend using it more often
Hey it's purely depend on length of content. But from my point of view you can keep keyword density of targeted keywords to 2-2.5%.
let me give an example of what i would consider the best for SEO This is an example of a blog post which focuses on the keyword "hotel door", and yes, nothing in this example makes any sense, that is not the point. I hope that made sense to anyone other than me... If you have a 400~ word article, include the keyword at least once in each paragraph, but only where it would be natural to put them in. The keyword should be in the heading and in the title of the article/blog post. as well as 1 or 2 links.
Yes, I agree. Keep the keyword density around 2 or 3% (you can use tools like Yaost to help you) AND do as the other poster suggested by keeping it flow and keep it natural. Don't be a keyword stuffer, Google will slap you ;-)