When you are targeting keywords.. .do you start with a small number and try to get some traffic form them and as time goes by keep adding more and more?
Depends on what you are talking about. If you are going to build a decent site, the yes. Start with your main keyword and build out the rest of your site using related keywords and synonyms. If you are talking about a mini site or landing page, then you could focus on your main keyword only, and build several pages around that keyword.