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 an audience of an event as merely numbers to be counted in order to assess the money they have paid for tickets. The earliest citation of it 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."