20 or more coins are okay but you need to watch them carefully for profit booking. I once held much bigger amount of altcoins but those were from bounty hunting and from other reward systems. So, how many coins you hold is not a criteria until you are able to track them and manage them.
Generally, I would say it's a bad idea to have 20 or more coins in your portfolio if you have bought them all with your capital. That's called over-diversification, and while diversifying your portfolio is a good thing, overdiversifying it can lead to problems, especially if the market starts dropping from where you've bought most of them, and it's certain that altcoins are mostly more volatile because if Bitcoin drops 5%, altcoins would usually drop a higher percentage from their value.
The best way is to have about 5 coins or so apart from Bitcoin. This way, you will have more money invested into each one and if one of them performs well and others don't, you will still get something out of your total investment capital and with Bitcoin, things will be balanced and you won't lose more value.