I'm gonna be honest, you're asking a question that really doesn't have much meaning. You've chosen undefined and arbitrary categories, and you didn't really define well the metric you're looking for. Originally I thought you wanted how many PLCs are installed across the world, now maybe you want the PLCs per facility?
The question is even more complicated by the fact that PLCs don't have uniform performance. You could have the same system controlled by 1 high end PLC, or several low end ones.
I'd be willing to bet that some marketing organization like ARC has asked a similar question, done the research, and published the result, if you're willing to pay for it.
The link below might be a good place to start:
https://www.arcweb.com/technology-evaluation-and-selection/plcs-plc-based-pacs-control-systems