There is some debate regarding this, NYU’s Stern School of Business’s Haran Segram says that you need a minimum of 20 stocks to be considered diversified. Many market participants may have as few as three or four stocks, which is insufficient to protect against market downturns. Generally, no more than 5% should be invested in a single stock. Otherwise, your portfolio is at risk if something happens to that stock.
However, you can simply invest in an ETF that is designed for diversification. You can also invest in several ETFs. This is easier than selecting individual stocks, which would take more time and expertise.