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.
Yeah... It should not just focus on 1 coin. As you said we will not experience much losses if not just focus on 1 coin. At least we have to have 5 coins or more in the portfolio, so when we fail in 1 coin (hopefully not happen) then the failure can be helped with other coin. In addition, the funds for investment also becomes lighter. Another advantage is when we are having financial problems we can sell one of the coins we have.