Honestly, OTC quantities vary depending on what is legal where you live. It has been 8-9 years since I lived in the US but nevertheless: At the time, you could buy 1000-2000iu strengths. I worked at a large pharmacy chain and stocked vitamins weekly.

Then I moved to Norway. One strength is sold in most stores - 400iu per tablet. You might be able to buy stronger in the pharmacy, but I had to have a prescription for a higher strength when I had low levels.

And to be fair, 2 of those store tablets keeps my levels high enough. So while 400 might be inadequate, 800 certainly isn't. Even without enough sunlight: I literally cannot get enough during the winter because the sun simply isn't strong enough when it happens to be up.