It depends on your objectives.
You can build some wealth by diversifying but the secret to becoming rich is being concentrated.
Most of the ultra wealthy of the world built it by being very concentrated (one company) and being right. Many more examples of being wrong than right obviously. This
is an article from NYU about not diversifying, pretty good read.
I think that NYU also had a class on this. The class focused on picking the one company you would invest in and diversification was a tool for those who already had wealth to manage risk. From what I read the class involved a ton of research into one company and if you lost everything by picking the wrong company it just made you a better investor because next time you would learn from the process and do better research. Seemed interesting.