This question: https://www.eia.gov/tools/faqs/faq.php?id=77&t=11 does appear in U.S. EIA site and seems to be quoted by other scientific publications.
"how much" refers to the volume of the emissions. "how many" could perhaps be used to refer to different instances or occurrences of emission, which is not the meaning that wants to be conveyed.
I would definitely have used "is" instead of "are":
- How much of U.S. carbon dioxide emissions is associated with electricity generation? (meaning: What volume/quantity of U.S. carbon dioxide emissions is associated with electricity generation?)
or, much better:
- To what extent are U.S. carbon dioxide emissions associated with electricity generation?