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According the State Dept., there are 9 million overseas Americans, not including overseas military service men and women (click here for Bureau of Consular Affairs overview).

614,553 (633,592 – 19.039) Uniformed and Overseas Citizens Absentee Voting Act ballots were returned in 2016, including 51,700 overseas U.S. military votes.

Military and Overseas Voting

• In 2016, 930,156 Uniformed and Overseas Citizens Absentee Voting Act (UOCAVA) ballots were transmitted and 68.1 percent — 633,592 UOCAVA ballots – were returned. Of the UOCAVA ballots returned by voters, 19,039, about 3 percent, were rejected. Of these rejected ballots, nearly half were rejected because they were not received by election offices on time.
• The number of ballots transmitted to overseas civilians increased by 23% from 2012 to 2016. Illinois, New Jersey and Washington are among the states that reported transmitting many more ballots to overseas civilians in 2016 than in 2012. Cumulatively, those three states accounted for an increase in about 40,000 ballots transmitted to overseas civilians.

https://www.eac.gov/news/2017/06/29/newly-released-2016-election-administration-and-voting-survey-provides-snapshot-of-nations-voter-turnout-registration-trends-voting-systems-election-administration-and-voting-survey-eavs-data-media/

The Military Postal Service Agency returned more than 51,700 voted ballots from military members to election offices (ballots are identifiable because of the kind of postage). The average military ballot return time was 5.1 days.

https://democracy.works/blog/2017/8/21/voting-abroad-fvaps-report-on-the-2016-election

About 562,853 overseas Americans voted in 2016. Since 4.5 million overseas Americans are eligible to vote in 2018 and 2020, the potential to grow the overseas vote is huge.