Everyone has different methods so it is hard to tell to be honest. I have many stocks but use a principle of wether the stock is stable long term first and then I use data as my trigger. This means I don’t need to do too much research as I buy dips and my general % increases.
If I wanted to make huge gains then yes I would need to know my companies inside out but I am not investing that way. Example if Tesco, BP or another big/stable company has a dip on a day I buy more of that stock. It generally works out but I do have to follow the news but not study full time.
I also have stocks like Berkshire that I don’t review at all as I know they generally go north. All in all I have a safe diversified portfolio but use 10-20% on riskier stocks that I pay attention to on the DD.
Obviously that is just my method but my point is everyone is different and only you can decide what is best for you. If I just did ETFs then I would put money in and forget which would be worse than being involved as now I am interested I find I put more in my account.
The % may be lower than an ETF, it isn’t at the moment by a long way, but the balance is higher as I put more in if I am actively involved. I find I don’t buy a coffee but a couple stocks instead
As for research I find the forum here a very good starter and then Yahoo finance for data and the news links on each stock below