I think it’s best to spread the investments out across 25 or 30 mutual funds with about 25 to 30% in safe funds like money markets, corporate bonds, and treasuries. Spread each of those across both short-term and long-term bonds/treasuries. The remaining 70 to 75% of investments goes to many growth funds in many sectors and different parts of the world (maybe 70% or so in the U.S.). The reason I like 70% in growth is because you need to be sure to outpace inflation in case you live many decades in retirement. The reason you diversify a lot is to almost always have some funds that are us even during severe market downturns. (I.e. my tech fund and health science fund and LT treasury fund did really well during the big stock downturn from mid Feb to late March.)
My portfolio is mostly Vanguard mutual funds and EFT’s. I spread the portfolio over ten funds. I would pare down the portfolio by two funds, but the two I want to sell have been in my portfolio since 2005 and the capital gains tax is the great procrastinator. At my age (61), my portfolio is not as aggressive as is was a few years ago. My big holding is the Vanguard Wellesley fund: 65/35 bonds/stocks; with an average annual return over the last five years that ranges between 6.07% and 6.54%.