When I saw the show, the only understudy on was Kiara Michelle Lee for Heather Duke. I believe Olivia Hardy has missed a lot of performances, going back even to early previews.
Lorna Courtney is currently on vacation this week and McKenzie Kurtz is on vocal rest. Casey Likes has missed a few shows for prescheduled absences, and Erin Morton was out for two weeks to work on a different project. And obviously Kerry Butler is on leave for Bat Boy.
The run was originally only slated to run through September 28, so it's possible that some cast members (like Butler) had pre-existing conflicts that are taken into account.
"You travel alone because other people are only there to remind you how much that hook hurts that we all bit down on. Wait for that one day we can bite free and get back out there in space where we belong, sail back over water, over skies, into space, the hook finally out of our mouths and we wander back out there in space spawning to other planets never to return hurrah to earth and we'll look back and can't even see these lives here anymore. Only the taste of blood to remind us we ever existed. The earth is small. We're gone. We're dead. We're safe."
-John Guare, Landscape of the Body