I think the point about the 15 is that it takes a lot of time to properly research companies enough to have the edge over other people, so I think it’s a compromise between having a small number so you can spend enough time to ensure they’re still quality picks and large enough for diversity in case one has big problems.
Specifically, he’s saying that if half the companies are just average because you didn’t research them well, they’re pulling down the rest of portfolio where you did put in the effort.
As you get to riskier companies, you need to diversify over more companies, because you might be looking for 500% growth in one company and the rest failing entirely compared to stable markets where you might get 0-20% growth across all of them.