5 eggs is normal for a cockatiel clutch... 12 means she's double clutching and seriously shouldn't be.
If you remove the eggs right away, she will try and fill out the clutch... best thing to do is let her lay all her eggs (should take about a week, either one every day or every other day) and then remove them all after she's done sitting on them for awhile... (normally it takes 3 weeks for fertile cockatiel eggs to hatch) they won't be fertile without a male, but you sure don't want her to get egg bound, and you don't want her double clutching or laying that many eggs at once. If she becomes egg bound, the vet will give her fluids, calcium, and something to help her pass the egg while keeping her body temperature up.. not cheap by any means... and all this may still not save her.
If she will eat cuttlebone, make sure there is plenty of that in her cage, if not, you can scrape it onto her food, or give her some bread, it has plenty of calcium in it and she needs the extra calcium to lay eggs.
If she continues to lay eggs, and it becomes a health problem, (which can happen at any time) the vet can remove her reproductive organs to stop her from producing eggs, but they may also just try medication to keep her from producing so many eggs.
There are a few things you can also do, you can limit the amount of light she gets every day which will decrease the interested in laying eggs, make sure you don't stimulate her by petting her tail area either which could trigger her mating/laying instincts...
Hopefully you can convince her she really doesn't need to be laying eggs...

------------------