If you have a site, say, howtoplaybanjo.com, and you've optimized it for the phrase 'how to play banjo' and you have high ranking in the serps for that term. If you decide to expand your site's scope to guitar. Does it compromise your banjo position if you start optimizing for guitar also? If you change the title of your home page from "How To Play Banjo" to "How to Play Banjo and Guitar" do you lose ranking for the search phrase 'how to play banjo"? What is the practical limit on how many keywords you can optimize for?