Diversification can only bring you further. But if you don't know what you are doing, like investing in shitcoins and coins that are hype by FUD and pumpers, you can't earn good profits. And if you over diversify your portfolio, heck, you can still get negatives at the end of the rope.
Just 3-5 coins is enough in my opinion. The key here is to invest in crypto smartly. Do your homework and learn how to enter the picture at the precise moment. Like buying in dips and not on top price.
So one question too: for coins hold long term is recommended on like cryptopia, Binance and cex.io? Or where should keep it all together on which wallet?
Keeping funds in a exchange is not a good practice. Because on a likelihood of a hack, your funds will be stolen. Much better to withdraw a few and keep it in your wallet that you have control of the private key and just kept a few on exchanges for trading purposes.