Its not so much the occurrence of a word, but the power of the word within the content. You could look at it as a percentage.
There are however a lot of factors to take into account. A word in a heading will carry more value, titles will carry a lot more value and text that stands out will also have more value when it comes to density.
Don't make the mistake of going all out go really boost your keyword density, this will only lead to an over optimisation penalty.
Carry out searches and see what sites rank well. Go over their source cone and note down occurrences of words and where they are placed.
There is no "right" number of times for a keyword to be used that will make your content more effective, so I wouldn't worry about keyword density at all. The key is to write content for your human reader FIRST and then naturally optimize the titles, headings, Meta descriptions and content to make it more "SEO friendly." But if you find yourself having to force a keyword into the content it means you shouldn't do it.
3 to 4 % keywords density is good for any webpage. I have checked website with 3 to 45 keywords density ranks well in search engine for targeted keywords. I think keywords placement in meta tags, h1, content and footer text is the perfect way.