How Many Capsules Of 50mg Vyvanse (lisdexamfetamine) Would I Have

How many capsules of 50mg vyvanse (lisdexamfetamine) would I have to take for it to be lethal? I sometimes double up or forget how many I've taken. I really want/need to know. Plus, I need to know how many to hide from my sister, because she might steal my pills since she's depressed, if I always hide just the right amount, she'll never suspect a thing and won't be able to overdose