It doesn't stack, per se, but it is good to have more than one.
Just like the scavengers, each ankh would only return 1 creature per turn. So if you have 1 ankh and the oppoenent kills 2 of your creatures, only the first one would get returned.
I was debating whether it should be destroyed or returned when a non-crystal creature dies. In the end I decided that it may be a bit on the UP side if it was destroyed...
As an alternative, I could lower the cost on the unupped version (and maybe even raise the upped cost by 1) then have the unupped version get destroyed whereas the upped version gets returned.