What is and isn’t a monopoly is of course decided on a case by case basis - as it should. There is no law saying that 80% market share is necesssarily a monopoly or that 20% is so low it can’t be blocked.
> Do you think a small mom and pop store in a rural captured market couldn't price fix?
Not sure how it’s relevant but they can of course haves local monopoly - but afaik in the US such local monopolies aren’t subject to regulation.
Can you show me the federal code that includes number of customers?
Do you think monopoly rules are about size or behavior? Do you think a small mom and pop store couldn't act in monopolistic practices?
Do you think a small mom and pop store in a rural captured market couldn't price fix?
Hint: It's not size :)