It seems like there are almost 500 thousand Argentinians with the right to vote, living outside Argentina.
About 100 thousand in the US, I believe.
This article shows how they voted last time around:
Un total de 435.556 son las personas argentinas que residen en el exterior y que estuvieron habilitadas para sufragar.
www.ellitoral.com
And it does seem like most of them went for Milei.
To be sure, turnout seems to have been low, maybe 5% in Madrid, but with a polarised, riled-up (something Milei does very well) electorate, and a close-run contest (like the Peru election I mentioned), there is the potential for an election to be decided outside Argentina.