My 2 cents.........

if your goal is to buy individual companies , then diversification is important. However , if investing in ETFs , then this is less important. Most Robo - advisors firm will put clients in portfolios with 10 + ETFs , why ??? I've been holding SPY in my RRSP ( Canadian 401K) since 2009, all 190K in it . the SPY gives me exposure to all 500 stocks of the S& P 500 , no need to be more diversify than that. Of course some people will hold some bonds ETF as well , which is good, but in my opinion , you should hold no more than 4 ETFs at a time ..... again , just my 2 cents.

From his Wikipedia page says

In 1999, during the dot com boom, Broadcast.com was acquired by Yahoo!
for $5.7 billion in Yahoo! stock.[25] After the sale of Broadcast.com,
Cuban diversified his wealth to avoid exposure to a market crash.[26]
In 2011

diversified his wealth to avoid exposure to a market crash
He also says he hardly buy stocks.

Diversification is like a two-sided sword. If you're extremely diversified, you will earn regular returns and often protected against systematic risk if you are smart about positioning. If you're extremely concentrated, you can often expect to earn large payouts and extraordinary returns if you are smart about specific selection albeit not limited to large drawdowns.

Lionel, in my opinion, you should diversify more than that. Practitioners focus more on tail risk now than anything, standard models are not robust enough to handle tail risk. It is no longer considered "diversified" to put your money in SPY, 500 stocks is nothing, it is how much you put into global portfolios. The benefit of diversifying into additional - and perhaps alternative - investments far exceed the cost of doing the trade.