For Federal elections, all of them.
Astronauts are covered under the same provisions as other US citizens residing outside the US. The Uniformed and Overseas Citizens Absentee Voting Act (UOCAVA), amended by the Military and Overseas Voter Empowerment Act (MOVE), requires that all states and territories allow US Citizens residing outside the US to register and vote in Federal elections.
State officials must...
provide UOCAVA voters with an option to request and receive voter registration and absentee ballot applications by electronic transmissions and establish electronic transmission options for delivery of blank absentee ballots to UOCAVA voters;
According to NASA...
Because astronauts move to Houston for their training, most opt to vote as Texas residents. Of course, NASA’s astronauts come from all over, so those wishing to vote as residents of their home states can work with their counties to make special arrangements to vote from space.
Once their FPCA [Federal Postcard Application] is approved, the astronaut is almost ready to vote. Like many great things in space, voting starts with an experiment. The county clerk who manages elections in the astronaut’s home county sends a test ballot to a team at NASA’s Johnson Space Center in Houston. Then they use a space station training computer to test whether they’re able to fill it out and send it back to the county clerk.
The astronaut will then cast their vote, and the secure, completed ballot is downlinked and delivered back to the County Clerk’s Office by e-mail to be officially recorded. The clerk has their own password to ensure they are the only one who can open the ballot. It’s a quick process, and the astronaut must be sure to submit it by 7 p.m. local time on Election Day if voting as a Texas resident.
If they cannot access a ballot, there is a backup Federal Write-In Absentee Ballot.
For local elections, the Department of Justice states...
...most states and territories have their own laws allowing citizens covered by UOCAVA to register and vote absentee in state and local elections as well.