When people talk about diversifying your portfolio, they do not mean to sell some of your bitcoin to buy a bunch of shitcoins. (Or, if they do mean that, then they are idiots and you shouldn't follow their advice.)

The entire cryptocurrency market follows bitcoin. When bitcoin goes on a bull run, the market goes on a bull run. When bitcoin dumps, the market dumps. Sure, there is the odd coin here or there which suddenly double or triples in value over a day or two for no good reason, but selling your bitcoin to buy shitcoins in the hope that you get lucky will result in you losing money 99% of the time. The whole point of diversification is to spread out in to different assets which are not significantly correlated (or even inversely correlated) with each other, so if one tanks, your entire portfolio doesn't tank with it.

You don't diversify your portfolio by buying 20 different oil mining companies, or by buying 20 different precious metals. Similar, you don't diversify your portfolio by buying 20 different random shitcoins.