Yes.. 10-20 keyword phrases should be fine if each of them has got considerably good amount of searches per month.
Just a note.. if you're willing to post your articles in top article directories, the purpose of writing articles should not only be getting inbound links from the article sites to your blog/lens, but also be driving targeted traffic to your site through those articles.
If your blog/lens is brand new and you can't wait a couple of weeks to get it rank higher in the Search Engines, you should try to optimize your articles in such a way that they rank in the top 10 search results (within days) and drive traffic to your site faster. :)
