Just 1.

If your goal is to hit Athena 10 quickly I strongly suggest "speed-running" your Athena Missions. (look on other posts for the best tips on how to do this.)

But my advice it just take your time and enjoy the journey.

@eskalibur7 you will get one per voyage...sometimes Rare will release "gilded Athena voyages", which will give you three Athena chests for one voyage but that usually only happens a couple times a year.