Our bachelor party had about 15 when we went, and there was a bachelorette party of similar size there on the tour at the same time. Twenty-ish people are easy to accommodate on the tour. The seating will be more snug, or spread between a few tables, but the place is intimate as is.
Are you going for the tour or are you having a party? Either way I think it'll be fine
You'll have to book the whole place
20 is doable if your group is the only one there. The tour includes a few drinks and $12 a drink there after. The tour guide is very entertaining.
They can definitely accommodate a group that size.
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