are 90-100 crunches too many crunches or just the right amount??
as much as you can do (I know a guy who does 700 a day: crazy :D), but if you wish to have a better workout, combine multiple abs exercises (5 or 6) and do 20 each, so you will stimulate your abs the most - so you will have the most efficient workout.
it depends. So long as you are feeling the burn, but are not injuring yourself then it should be fine. I do 100 so I think its fine :) just do like 25 deep breath 25 more deep breath 25 more deep breath then a final 25! It should be good!!!