I did some tests to see the effect on the total volatility in the portfolio by reducing the number of stocks I hold. The small test gives some indications to not have a very concentrated portfolio (below 10 stocks) . When testing a lot of timeframes with 10 stocks or less, the volatility in performance and randomness are very high.
Here is my EU portfolio:
Here is my US Canada portfolio:
Im not sure there seem to be a big difference between the US vs. EU.