FTM and formula

Jessica
So I'm out looking today at the prices of formula. I got to thinking that i don't honestly know how much I'll need to buy at first. How often a infant goes through one can of formula that is 21.5 oz.? How much do I need to buy during a shopping trip. 
I guess my question is, how long would a 21.5 oz of can last? And how many do you usually buy during a grocery trip? 
Picture for example.