Check on the bag for feeding quantities. Whatever is written there, that's the dailay portion, devide it to how many ever meals you want (2 is good though). She probably lost weight just being at the shelter, bored and sad. I'm sure she'll be fine.
My Chloe, who was never at a shelter (I got her from a breeder in MO) is a skinny little girl. I tried getting her to put on another lb or 2, but she won't. She is not too thin though.
As long as you cannot SEE her ribs, but can feel them when you pet her - she's fine!