I am not sure this is a god strategy, I have 5-6 coins, and try not to keep more than that as it becomes impossible to keep following the coin's news.
Is holding up to 5-6 coins only too risky ?
how many do you hold and what is your rationale for that number ?
Some people spend 24 hours a day trading and so actually manage to follow 10 or even 20 coins. You should keep as many coins as you feel comfortable with.
I hold BTC because it is the most established coin and few more alternative coins: Litecoin, Peercoin, Namecoin, NXTcoin. That makes 5 coins for me. Choose the perfect number for yourself. Don't rely on the opinion of other traders as they all have different strategies and spend different time on research and trading.