A well-diversified portfolio needs about 20-25 stocks from different categories.

Correct.

The ideal number of stocks to have in a portfolio is entirely dependent on a number of factors like country of residence, your investment time horizon, and your prosperity for reading market news. Therefore, there no single correct answer to this question. However, investors in the United States, where stocks are less correlated to the overall market than anywhere else, the figure is about 20-30 stocks.