How many stocks should we have in a portfolio?

The STI index is made up of a basket of 30 stocks. The Dow Jones Index is also made up of 30 stocks. Usually anything more than 30 would dilute one's returns and become insignificant. Anything under is fine but definitely not over otherwise you'd be better off buying an index

It should not be a question of how many stocks you should have, but how many sectors you should invest in.

investing in different sectors will diversificate your portofolio and reduce your risk exposure

Why not invest through index funds as well, and own 2000+, 10,000+ stocks in total?

There is no additional commissions to passively investing through index funds, so I guess the question you may want to ask yourself is how confident are you in hearing an index consistently over the long term with minimal cost and effort?

Well if you are just starting out, I would think 10 is the max, so 10%/stock. Once you have gain more knowledge and experience, I would recommend to reduce that amount to about 5-7, of course by then your conviction level should be strong to be able to allocate more $. With that said, a rule of thumb is to not exceed 30% holdings for a particular stock. There is always this risk of a black swan event that no one knows if might happen.