Generally in order to target for high paying keywords ads to appear, try to focus on writing a content based on the keywords itself. Some people have mentioned also that limiting the number of ads does increase the chance for high paying ads to appear but I'm not sure how effective that is. The only way is to try out and see for yourself. However, one thing for sure is that placement does not comes into play over here.
YES... The lesser the number of ads, chances are people will click only higher paying ads.. If there are 10 ads on ur site the first spot if usually where the highest CPC ad is served.. the last ad could be as low as 1 cent. SInce an user will click usually only one ad, it better to only serve him high playing ad by limiting number of ads .. Unless you have a very popular site with very good niche