How should a long-term, high-risk investor diversify and rebalance?

[Kenneth_R]

I'm in my late twenties and finally in a position to start seriously investing for the long term, but I'm paralyzed by the concept of portfolio diversification because every article I read seems to contradict the last on the ideal asset allocation for someone my age. I've maxed out my retirement account contributions, which are in target-date funds, but I now have a separate brokerage account with a lump sum to invest and I'm terrified of putting too much in a single sector or asset class without understanding how they truly interact during market downturns. For experienced investors, what is a sensible, foundational approach to portfolio diversification for a novice with a high risk tolerance but a long time horizon? How many different ETFs or mutual funds are realistically needed to be properly diversified, and how do you rebalance your portfolio without incurring significant fees or overcomplicating the process?