It is inevitable that, over time, you will have some big winners in your portfolio and some big losers. By diversifying, you are also spreading the risk. A one share portfolio is a risky portfolio. If the company doesn't progress as you expected, you could be left nursing a sizeable loss. Like investment strategies, we could debate the merits of diversification until the cows come home.
Some people feel comfortable with a small number of companies in their portfolio.
When you consider the amount of money he has invested, Warren Buffett has a relatively small number of companies in his portfolio. When he buys, however, he buys big, taking large positions in companies he considers have outstanding prospects. Buffett says that truly great companies are rare, and when you find one available at an attractive price, you should buy up big. Some people like to own shares in many different companies. I know of some people who hold shares in about 30 or 40 companies.