I was surprised to see this, I went into my 'cart' to try and update the number to 50 for a case and got the message again, "due to the extremely high demand for... limit is 20 boxes per customer".
I'm surprised they did that also, but I can't say that I blaim them. They're trying to be fair to all of their customers and that's a big challenge these days.
At least they're notifying you on the website... I ordered 1000rds of 9mm from Georgia Arms a couple weeks ago, and only 500 showed up, with a note on the invoice.