Statistics like this ignore one aspect: how well can you understand and follow up 60+ different companies? That introduces a new risk: let's call it ignorance risk.
In the same vein: how is your tenth best idea going to be vs. your best idea? My own bandwidth is limited, tracking 10 companies is quite a cognitive load already.
So it depends on your level of involvement, I'm pretty binary in that perspective. If you are highly involved, concentration is a benefit. If not, it is a liability and you should stay away, and go for a broad index.
An extreme example is owning your own business: highest involvement, high concentration yet can still be a great idea.
this is where i differ from most
first of all i hold over 200 stocks
how do i track them all ?
I DON'T !!
what i do is if a stock is up ( say ) 150% i sell down taking out the original investment cash ( and costs ) the aim is to sell 40% of the holding and keep 60% effectively letting the profits run , and reinvesting the cash elsewhere .
i inherited a quartet of stocks in 2011 at the start of my investing adventure and one stock dwarfed the rest of the portfolio close to 40% initially since i was still working at the time , i put surplus cash into other stocks ( specifically MQBKY .. well the ASX listed version of it )
i brought up MQBKY up to 30% and when in sufficient profit reduced swapping into BHP ( now the 3 rd biggest holding just below 5%)
current my biggest holding is still 10% with MQBKY at roughly 5% but these two stocks ( and about 30 others ) has no investment cash at risk in them .. 100% profit running for me
so i have over 35 stocks at absolutely no ( cash ) risk , some stocks that rarely traded , but pay dividends ( one little gem has paid me 110% of the purchase price in divs in just 6 years )
and lets face in my main aim is ( retirement ) income but with some attention to asset growth ( preferably via extra shares courtesy of selected dividend re-investment )
yes i am aggressive in this approach but the plan was to retire at the start of 2020 , but fate decreed i cease work in 2017 , i NEED to make that money work hard just in case i live past 2026 .
cheers !!