Don't limit your search engine campaign to 20, 30, or even 500 keywords. Having more keywords is better because you can discover which inexpensive keywords are working over time. You should start with a massive amount of keywords and filter the ones that do not generate sales. This helps you to minimize waste by cutting your bid price and generate more income from sales.
Continue to add specific terms and filter them out, both through Google and your own market testing. The key to the entire operation is having well written ads tested for actual sales conversion. You may test these conversions by adding third-party tools in order to track actual shopping cart sales.
When refining search engine marketing plans, there are always more keywords and untapped markets to add to your Google Adwords campaigns. It will require continual research and experimentation to add keywords drawn from new markets and keyword categories.
Avoid keyword stagnation with your search engine campaigns.