What is the average number of words necessary to read classical Chinese?
According to 二十五史的字数是多少？, most of the classics contain five to six thousand unique characters. 《辽史》(Liao history) contains as few as 4071; 《清史》(Qing history) contains as many as 8080.

《史记》93 BC contains 5122 unique characters;

《汉书》83 AD, 5833 unique characters;

《新唐书》1060 AD, 6346字;

《清史》1927 AD, 8080字.

According to this site, the first dictionary 《说文解字》which was finished in 121 AD contains 9353 characters; another one which was created in 1716 AD contains 47035 characters.

Han dynasty's《说文解字》 finished in 121 AD, contains 9353;

Song dynasty's《集韵》(1034AD) contains 53525;

Qing dynasty's《康熙字典》(1716 AD) contains 47035;

《中华字海》published in 1994 AD contains 85000.