I found out that the greatest number of mice killed by one cat was 28,899. Her name was Towser and she lived in Scotland until she died in 1987, at the age of 21.
That is 4 mice per day on average. The best I've ever done was 3 mice in one day but then I am only 2 1/2 years old and am still honing my skills.
Nevertheless, I am so jealous!