...45. It didn't really benefit me in the end, though. There were just a few single empty seats near me and so I stayed where I was. There were a lot of solo travelers who ended up with two seats, though.
As of the night before our flight, the seat chart showed 65 empty seats.
However, FWIW, the flight to Heathrow that left just before my flight was oversold and they were looking for volunteers.