First of all , your main investment should be in Bitcoin and then you can focus on altcoins like ETH , Monero , Dash , Crown , Waves and so on but you should understand the project itself before buying into it. Right now , altcoins are very attractive as price because some of them are near all time low.
Bitcoin is one of the best coins so it's more important to hold bitcoin than other altcoins, at least we have 70% investment in bitcoin and the side of altcoin that you believe is the best altcoin at a price that will be higher due to the bullrun.
Well, the percentage depends on each person's intention. Personally, I only invest 50% of my capital in BTC, I will allocate other 50% into at least two different altcoins that I feel safe. Right now, I choose to invest in BTC and ETH only since the market is recovering and the price of those two coins are getting higher.