I brewed for a wedding last May. 100 guests. The groom was a fellow homebrewer so the homebrewed beer was a big focus at the reception. The guest gifts were tasting glasses.
Anyway, the only other beer offering was Butt-Lite and there was also wine and spirits at the open bar. All that said I took 40 gallons, 8 cornies. The 100 guests drank it all. There were a few pints I dumped at the end of the night, but 40 gallons for 100 guests was spot on.