Can the new 2019 16-inch Macbook Pro support 8K resolution?
You cannot just assume that an 8K display can be supported just because it supports 4 x 4k displays - it doesn't work that way, I'm afraid.

As far as I know, no 8k monitors with a single cable solution exists today. That would require DisplayPort 2.0, which is coming with monitors next year. 

So yes, at the moment 8k resolution through one cable is impossible.