Mexico – about1.6 million Americans reside in Mexico, most of whom may be Mexicans by birth right, but born in the U.S. or naturalized in the U.S. This group accounts for 27% of Americans living in another country, say some sources.
Canada – about 740,000 Americans live in Canada, but again, many are people with dual citizenship, or moved to Canada for jobs and now have permanent residency or married a Canadian. This is 12-13% of Americans living broad.
Combined, Mexico and Canada account for about 40% of all Americans living abroad.
India and the Philippines – about 1.5 million Americans live in those countries, but many are dual citizens, having been born there or born to parents who were native of those countries. The rest are usually there on a work assignment.
Those 4 countries may account for 27% + 13% + 26% – or 65-66% of those who live abroad.
The best estimate of how many Americans live abroad is about 5.4 million, with an often quoted 9 million estimate from the U.S. Department of State. The latter, however, includes some Americans who are currently traveling abroad and have not actually moved abroad.
Another large group is American military personnel and their family members – at 1.2 million.
Countries With the Most American Emigrants – WorldAtlas
From my own data analysis of 150 “news stories” about Americans moving abroad, about 1 in 3 have pre-existing dual citizenship or a right-of-descent ancestry immigration privilege. Another 1 in 3 obtained their residency or citizenship via marriage to a native of the country. That’s about 2 in 3 had an immigration privilege. See Most Media Stories of Americans Moving Abroad Are Misleading – Coldstreams Travel and Global Thinking
That sample, of course, is not a random sample – it simply counts the subjects of fluff news stories about Americans who have moved abroad.