[jack_h]

If five companies competing is anti-competitive, then how many competitors does it take to suddenly become competitive and why? What is the significance of this larger-than-five number? Is it the same across industries? How do you derive it?

This seems to rest on the mistaken belief that a corollary of monopolies being bad is that more competition is always better than less competition. If everyone was a competitor in the restaurant food delivery market we'd all starve to death as no one would be growing food. An efficient economy wouldn't waste resources competing over less important things like restaurant food delivery over something more beneficial.

[skt5]

The HHI [0] attempts to quantify that. [0] https://en.wikipedia.org/wiki/Herfindahl–Hirschman_index

[harimau777]

Perhaps the issue isn't whether there is a specific number of companies. It seems to me that there is a competition problem when companies behave in a way that is harmful/disliked/bad but no one steps in to provide an alternative because they can't compete with established marketing, economies of scale, network effects, etc.