In this edition of The Motley Fool's "Ask a Fool" series, Motley Fool One analyst Jason Moser and Motley Fool Stock Advisor analyst Brendan Mathews take a question from a reader who asks: "I've read your advice advocating owning at least 15 stocks. Right now I have six. I plan on investing a few hundred dollars a month. Am I better off putting that money into new stocks or the six I already have?"

Brendan suggests that if you find new stock ideas that you like, then it would probably make sense to add new stocks to the portfolio. Owning at least 15 stocks ensures at least a minimum degree of diversification, though owning perhaps 20 to 25 stocks would increase the benefits of diversification. Jason points out that it's also worth making an effort to keep costs down -- i.e., below 2% of the portfolio.

See more in the following video.

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