I'm not trying to wriggle out of this, but even with a quick Google search you will find (mostly non-scholarly) articles that tell you that you can can reduce unsystemic risk by about 70% with about 30 shares, even if thats all you invest in. That goes down again when you also invest in other asset classes.
I came across this view when I studied Investment Strategy as part of my MBA.
The articles where free when I studied, but sources such as Emerald Insight, JSTOR and even Wiley Educational are rarely free to non students. Try Google Scholar if you are interested.