Diversification is not an end in itself, but rather a way to mitigate portfolio risk. So the question of "How many stocks should I own?" will be different for everyone … but still, there are some good principles to think about.

Eddy Elfenbein, who writes about investing at Crossing Wall Street, is known for being a buy-and-hold investor -- CNNMoney named him "the best buy-and-hold blogger" on the Web -- and on his site, he has a public portfolio of 20 stocks. Last month, during an hourlong interview in Washington, D.C., I asked him whether he thought 20 stocks made for a diversified portfolio. Watch the following video for our exchange. (Run time: 1:40; a transcript follows the video.)