what does it mean when they say only 1 in 8 cases have been reported in the U.S.? It might mean that 103,200,000 people have had or presently have coronavirus in the U.S.

 12.9M cases so far. So, if I'm reading this correctly we would 

have to multiply 12.9 million cases by 8 to come up with this figure

one hundred three million two hundred thousand people have or did have coronavirus

so far in the U.S.

But, if this is true it means that 1/3 of the population (approximately) have had or do have a coronavirus infection.

But, this also doesn't mean people cannot get this infection more than once. This has already been proved multiple times in Europe and Asia already.

20% of coronavirus cases are serious enough to have to go to the hospital and around 2.5% of people tend to die from it if they can be intubated or 5% to 10% of them die if they cannot go to a hospital for one reason or another.

So, obviously there are more questions than answers here at present.

Note: The population of the U.S. was 328.2 million in (2019).