Heh, good find. It's never happened before. The previous record for the least number of players used in a series of at least three Tests was 23, which had been achieved twice: Aus (11) v Eng (12) in 1882/3, and NZ (12) v Aus (11) in 1981/2.
There have also been seven series in which both sides used 12 players, all of them in the 20th century: NZ v WI 68/9, Eng v Pak 74, Eng v Pak 78, NZ v WI 79/80, Aus v Ind 80/1, Pak v Aus 88/9, SL v Aus 99.
All of the series mentioned above were three Tests long.
An honourable mention goes to Eng v Aus in 1989 (my favourite series), which I think has the greatest difference between the two sides - Australia used 12 players, and England used 29.
no subject
There have also been seven series in which both sides used 12 players, all of them in the 20th century:
NZ v WI 68/9, Eng v Pak 74, Eng v Pak 78, NZ v WI 79/80, Aus v Ind 80/1, Pak v Aus 88/9, SL v Aus 99.
All of the series mentioned above were three Tests long.
An honourable mention goes to Eng v Aus in 1989 (my favourite series), which I think has the greatest difference between the two sides - Australia used 12 players, and England used 29.