No. of words per page
There isn't really such a thing as an optimal number of words per page.
However, it is known that having your relevant key phrases towards the top of the page is beneficial for search engine rankings, as well as being beneficial for visitors - see section Visitor Behaviour.
Thus you should feature your relevant phrases "above the fold" (visible on the screen without having to scroll down), with a sprinkling of these and their variations throughout the rest of the text.
