Mine says the same thing, but I get several doses from each vial. DH says that it probably says that because in a hospital setting, the MTX would probably be given at higher doses to cancer patients.
We keep mine in the fridge, all sealed up. Jeff always uses an alcohol swab to clean the top of the vial each time. He also uses a swab on the site that he is about to "shoot".
Relax, sweetie. It's not as scary as it seems.