The other thread going on this topic says that Europe could sell about $10 trillion. That’s a lot, but also, it’s only a bit over 10 days trading at normal volumes (according to the numbers being discussed in this thread).
For reference, 25-30% of US treasuries are foreign owned. It's still a lot but I think people over-estimate how much of US debt is foreign owned.
The normal trading volume isn't really the key. Rather, it's how elastic the price is.
Suppose these guys sell 10% of the daily trading volume. How do the traders in the market react? One possibility: Buy at current prices. Another: Speculate that there'll be more sales and the price will drop by a couple of per cent in the coming days/weeks, and delay their buying in order to buy the dip.
I'm sure the Americans have laid plans for how to avoid a major Oops.