Bums on seats
The paying audience at a venue with seating, usually a theatre or cinema.
This rather derogatory term was coined in the UK in the late 1970s/early 1980s. It indicates a view of the audience for an event as merely numbers to be counted in order to assess the money they have paid for tickets. The earliest citation of the expression that I can find is from the Guardian Weekly, December 1982 (although I suspect it is somewhat earlier than that):
"E.T. thus comes to a beleaguered industry like a gift from the gods. Not only does it get bums on seats but it encourages the kind of shared enjoyment that suggests the cinema still has something unique to offer."