The Holy Grail principle is that our main goal should be to find and combine the least correlated asset classes we can find. If you do this, you can reduce your risk by half or even more without sacrificing much at all in your expected returns. You'll get the most bang for your risk-reducing buck if you have at least four or five. And as Ray Dalio says, you could have a thousand different assets with 60% or better correlation but it's not going to reduce your risk very much.