You shouldnt only target 1 keyword, even if you have a very small budget or think that 1 keyword covers everything that you do or sell. This is a mistake because unless its a highly searched for keyword, and if it is then the competition to rank well for it will be fierce and getting your site in the top positions for it very difficult, then ranking for that 1 keyword wont attract enough visitors to your site.
You also shouldnt target lots and lots of keywords 30, 50, 100, etc. as the more keywords you target, the less resources you can assign to working on each of them, and the harder it becomes to rank well for any of them. Whilst you do ultimately want to rank for many keywords, to start off you need to be focused and realistic, especially if you dont have a big budget for SEO or lots of time available to create content and promote your site.