I don't think there's a magical number of competitors that fixes things. We see an interesting phenomenon in commercial landlords where we have a ton of players but they all use the same software [1], which creates an effective monopoly even though it's not a classical monopoly.
The problems we see in so many sectors were accurately described in the 1800s by just analyzing the workers relationships to the means of production. Internet access is a textbook cexample. National ISPs should not exist. They are rent-seeking parasites. The best Internet is in places with municipal broadband.
It's just not feasible or economical to have 4+ broadband networks being built in an area. [1]: https://www.propublica.org/article/yieldstar-rent-increase-r...