Normal distribution with 5% probabilities

The normal distribution in finance

All you want to know about the normal distribution in finance before investing. A gentle but rigourous introduction to quantitative finance.

How many stocks should I put in my portfolio ?

TL; DR: Not only one stock. Invest in 5 stocks to reduce your risk by half vs one stock investment (Result without optimizing portfolios, i.e., picking stocks at random and weight them evenly).