For a while now the EU 4G level has been about 50% of the total, and the US 4G about a third of that, so about 17% of the total. The rest is SE units. So if 1800 units in total are preordered, it looks like there'll be something of the order of 900 EU 4G chips needed, but only 300 US chips. To get 500 US units at these rates you'd need to get about 3000 total preorders.
The better news is that at the moment, it seems to take about 5 days to shift 100 preorders, so if things stay as they have done so far, it would take just two months (TM?) to reach 3000 preorders. Whether interest will last that long with new people still discovering the project remains to be seen though.