Not sure whether this is a definitive figure, but Simon Inglis’ Football Grounds of Britain gives a figure of 10000, including 2796 seats.
It also gives Glanford Park as 11226 (4800 seats) before changing to 9183 (6410 seats) when seats were installed in the away end.
I can’t find a figure in John Staff’s centenary history, but he cites an attendance of 7178 for the cup tie against Sunderland as the highest crowd in the final season at the OSG.
