It is better to have few, like 5-10, because if you have more, you've got to keep an eye on each of these alts. For me it is wastage of time.
I agree with this. I think having 5-10 altcoins would be enough. Choose carefully. I would suggest buying 5-8 from the top 20 on the CoinMarketCap, and investing in 2-3 new altcoins which can grow big time but also have a chance to disappear forever. Do not invest a lot in the new alts, but do not ignore them either. Who knows, maybe one of them will rise 1000 times in the future making you rich (which will hardly happen with the coins from the top 20).
I would advice getting about 15 altcoins, all belonging to the top 100 of the "best cryptos" however, not necessarily they need to be from the top 20 as there is a lot of potential on some other less known cryptos.