Flash 8, if you nudge forward to 1990, BA had the following types in service:
A320
BAC 1-11
146
ATP
B737
B747 (-100, 200 & 400)
B757
B767
Concorde
Tristar
DC-10
So eleven
I suspect a sometime during the 1970's Aeroflot would be the winner in this category
Don't remember us having any 146's when I was there in 1990, was there a franchise operation on it back then ?