I think between 50 and 150 are normal. Some people shed even more.
You can shed excessively if you have a poor diet, lack vitamins, are stressed ( I read than you will shed 3-4 months after a stressful event. That is the reason you are not always connecting the two things) or sick.
If you are worried talk to your doctor but unless you are pulling handfuls out he is probably going to tell you that you and your hair is normal.
-Katrine