Quite a few :)
pretty much everyone besides a few thousands (maybe) Korean, can I understand it like that?
Why is it 루마니아에 instead of 루마니아에서?
It is 로마니아에 because, as far as I know, words 있다 and 없다 are used only with 에 and never with 에서.
I think because foreigners don't come from Romania, they come from their home country, but they are in Romania. If it was how many Koreans are from Korea, then it would've been 한국에서