Here is the UN’s 2015 International Migration Infographic, which claims that there are 4 million “North Americans” (which would include Canada) living abroad.
Meanwhile, AARO’s estimate is 8 MILLION Americans (from the United States), twice as many as the United Nations has reported for both the United States and Canada. According to their website, they got this figure from the U.S. State Department.
This discrepancy is most likely due to the fact that AARO works with “guesstimates”, while the United Nations is only taking into account the officially reported number. In reality about half of Americans living abroad are not residents of their host countries nor registered with the United States Embassy, for example. In fact, most countries have a large “undocumented” migrant population, so it is likely that across the board the actual numbers of international migrants are much higher than those reported by the UN.