Appriximately how much formula do babies go through in their first month of life?

I'm pregnant with twins and have a medical condition that makes me unable to breastfeed so I have to bottle feed my twin daughters when they are born. I was going to buy their first few weeks to a month of formula before they arrive(I'm 32 weeks pregnant) but then it occurred to me I don't know how much they will go through. Thank you.