You can get substantial diversification through mutual funds and ETFs, but it is good to have increasing amounts of diversification the larger a portfolio is.
Here are some general guidelines:
If your portfolio is less than $50,000, probably 4-5 Mutual Funds will suffice.
If your portfolio is from $50,000-$100,000, you might consider adding a few more exotic Mutual Funds or buying a couple of ETFs.
If your portfolio is from $100,000-$500,000, you might want to hire an investment advisor or have 10-15 Mutual Funds and some ETFs.
If your portfolio gets to be well over $500,000, you should probably build a team of advisors, including a CPA, tax attorney, a money manager, real estate and insurance professionals, and so on, because you may benefit substantially from coordination between these advisors to optimize your financial picture and decrease exposure to taxes and possible liability issues, possibly through the use of trusts and LLCs to protect your various assets.
Do I Need a Financial Advisor?
Where do I find a Financial Advisor?
Do I Need Professional Help to Prepare a Trust?