In terms of not being able to read, I would count 8% as "many"? Select a group of ten random Americans and you'd expect one of them to be illiterate - that's appalling.

Right there you have 8%. But it is worse because it says they're able to "read to varying level", which includes 2nd grade level, or barely at all.

Over 20% of American adults have a literacy proficiency at or below Level 1. This is defined as "difficulty using or understanding print materials" and "may struggle to understand texts beyond filling out basic forms." So 1 out of every 5 Americans is functionally illiterate. I would say that counts as "many of them don't even know how to read."