[header] How to buy wine for a crowd [title] Calculate how much wine you'll need. [substeps] The average guest will drink 2 glasses of wine per hour, with each pour being about 4 oz. ( 120 ml). 
Multiple the number of people attending your party by 2 (2 glasses each per hour) and then multiply that number by the approximate length, in hours, of the party. Therefore, if you have 20 guests, they'll consume 40 glasses of wine per hour.