There seems to be a set number of Earthen. They talk about making new Earthen in questlines & when you get the machine that makes Earthen up & running & unlock the allied race, the starting questline talk about how each new Earthen is simply an old Earthen given a new body. So they're functionally immortal in a completely-bypasses-the-shadowlands kind of way. Although that's strange because they have monuments that serve as epitaphs to Earthen.
Seems like the actual "essence" of an Earthen is a core crystal that can be installed into a new Earthen, but if that gets destroyed they're actually dead.