The short answer is one. Or three.
You can buy just one fund if you can settle for a preset mix of stocks and bonds. Balanced funds, which generally hold about 60% of assets in stocks and 40% in bonds, can suit a broad range of investors. If you are buying just the one, pick a fund that diversifies among many blue-chip U.S. companies on the stock side, with a conservative mix of U.S. Treasuries and high-quality corporate bonds in the fixed-income portfolio.
Target-date funds offer a combo of stocks and bonds that gradually changes as you near your preferred retirement date. For example, you might buy a fund with the date 2030 in the name. That fund could hold about 75% in stocks today, but go down to about 50% in stocks at retirement time.
If you don’t want to leave your stock and bond allocations up to someone else, you can build a low cost portfolio that own most of the global market with just three funds. A “total stock market” index fund will hold over 3000 stocks, ranging from small companies to established corporate giants. Round that out with an international index fund to cover foreign holdings, and bond index fund. You can get fancier that, but you don’t have to.