Not that few actually but it all depends on config. Most airliner have business class up there so not a huge amount of seats. For example BA
747-400s have 20 seats and CX
747-400s has 22.
Corsair has mixed business and economy upstairs in a 2-3 for biz (pretty tight compared to a traditional airline) and 3-3 for economy. 24 business and 18 economy.
If you go all economy on a -400 you can probably squeeze in 13 rows, which is 78 people. The -8 will have a longer upper deck so more sardines.
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