For an open bar consider using .75 glasses per hour as your wine consumption rate, so for a five hour party, you would need to add in an extra two hours of consumption. This would mean an extra five bottles.
(20 people x 2 extra hours x .75 glasses per person = 30 glasses/ 6 glasses per bottle = 5 extra bottles)
If you do not have an open bar and your guest are enthusiastic, consider using 1.25 glasses per hour as your consumption rate, for an extra eight bottles of table wine.
(20 people x 2 extra hours x 1.25 glasses per person = 50 glasses/ 6 glasses per bottle = 8 extra bottles)
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