Is there any
testable proof that one, two, three or four keyword terms per page is optimal ?
A page should be able to show up for multiple terms. Dozens of terms. If a page does not, then it is missing a lot of opportunity. Remember, *most* searches are unique.
Check out the Long Tail as to the reason why this theory is sound.
http://en.wikipedia.org/wiki/The_Long_Tail
There is more traffic in the tail than in the head. i.e. if you collectively target the lesser known phrases, you'll end up with more traffic than the guy who only targets the top phrases.