No they don't stack and that is very annoying. I never used to use them either, but a few weeks ago I activated one for some reason and found that it does make my life easier. As I have tons of them too I've been running them constantly since then. It means that a regular tournament round, or collecting from my May Trees, ToS, Stonehenge, never overflows it. The only way you can tell if it is going to run out is by going into your spells menu and hovering your mouse over the icon.
Like @Sir Derf I still have quite a quantity (maybe 250?) so it would be good if they would stack or just auto-activate... or at least show a timer.