i think it depends on kind of your website . if you have a site with concentrated keyword like tutor website . you just have some of key words like tutor. finding tutor math tutor...... but if you have a website about books . you have a lot of key word as a name of book
its depends in what volume and how many link building techniques you are using.. for example if you want to boost you ranking by using on article submission then i'll suggest you to target 3-4 keywords only..