Well, that's a relief! I thought I missed the mass migration from the US to Argentina.It was a bit of a joke. The population of Buenos Aires is around 15 millions. And technically speaking Argentinians are americans, like Mexicans, Chileans, Panamenians...
Using American to refer solely to the people of the United States can generate some confusion down south (I know it for a fact)
Seriously, though, I've never been able to find anything close to a reliable number of Americans in Argentina. The government must (You would think?) maintain the number of immigrants and residents from each country. I've found a reasonably reliable source for the number of immigrants (about 5,000 from the US), but I've never seen a number for residents.