Just like stock, you can invest all your money for a coin code if you really trust. But that would be very dangerous. One can make sure that the coin will not fall down to 0 a day.
The advice is to diversify your portfolio. That will help reduce the risk. You do not earn much but if not lost also much.
I have to disagree with you. There is no point in diversifying your risk by investing in more altcoins. Because if Bitcoin goes down, then the alts will also become worthless. If you want to diversify your portfolio, then invest in some other asset, such as real estate, equities, or mutual funds.