I disagree fundamentally with that. a minimum of 10 coins. just find the coin price statistics from the top 15, period 2017-2018 and you will change your point of view
I agree that back in the 2017-2018 period holding a variety of coins/tokens was the best strategy. I would've lost out big if I'd chosen only 3 back then. It was an explosion of speculation and we were all in the honey-moon phase with cryptocurrency and everyone was taking their first steps into the crypto-market, and no one could do any wrong. Every move was the right move.
But then time passes by, people cool down, crypto is no longer new and shiny, and we've seen how every cryptocurrency is capable of crashing hard.
I do not believe we're going to see the exact same thing happen again 2018-2019. Everyone is much more sober this time around, and I think the next phase coming will weed out the lesser coins. Using past behavior to predict the future is not the wisest strategy here. If anything, 2017-2018 showed us that past behavior is not an accurate indicator for future trends.