Common stocks represent the ownership share in a specific corporation. If your portfolio is composed of one single stock, called Alpha, there would be two broad sources of risk. At first, your portfolio would be subjected to the uncertainty coming from the conditions in the general economy. For example, inflation and interest rates could have an impact on the performance of your stock Alpha. In addition to the conditions of the general economy, you must also consider that the factors depending specifically on the company's management will also affect the rate of return of the stock Alpha. If you include another stock called Beta in your portfolio, for example placing half of your funds in Alpha and half in Beta, the diversification will reduce your portoflio risk (as long as the specific risks of the firms differ).
If you add several different stocks in your portfolio you will definitely reduce your exposure to firm specific factors and the portfolio risk. Diversification can reduce risk to very low levels because including several stocks in your portfolio will reduce the exposure to any particulare single stock. However, risk can't be specific only. There are common sources of risk that affect all firms. Even if we are able to build a portfolio with very low specific risk, we must remember that it is impossible to reduce the risk to zero because there will always be a systematic risk.
Systematic risk is an uncertainty that cannot be reduce to zero and it is the risk that remains after we build a well diversified portfolio. You should never invest in one stock only because the specific risk of your portfolio will still be high. Investing with exchange-traded funds will give you a great exposure to several stocks and at the same time you will consistently reduce the specific risk of your portfolio.