Charlie Munger wisdom I have never bought into the diversification story other than to buy industry specific ETF's where my knowledge is lacking.
If someone is willing to do the work and has a modicum of skill, they can get to know about 3 stocks pretty well and another couple of stocks to the point of being usefull. When I was an analyst, I had all of the tools and connections at my disposal and it was still impossible to get inside more than a couple of stories. The amount of time sensitive pressure on analysts is staggering imo and as such leads to an unhealthy lifestyle. Eventually, analysts can cut sweetheart deals to migrate to the buy side side where they can breathe and actually become useful.
The benefit of knowing a company well is that you will likely be able to evaluate the impact of both micro and macro economic events on the company.
Here is a clip from the recently deceased Charlie Munger that is very accurate imo:
https://www.youtube.com/shorts/VEyM3r-tAIw