The fact is, we will never know for sure. In "Statto's Column" in the Spring 2004 Brighton and Hove Albion Collectors' and Historians' Society newsletter (no. 18), they split the difference at 7,500:-
Three different media outlets have recently reported the Albion fans as numbering at least 10,000:-
SEAGULLS TV: MULLERY ON NEWCASTLE 1979
The History of Brighton and Hove Albion v Newcastle United
Albion staging 40th anniversary promotion reunion at the Amex