Despite this long period in time, there have only been 14 world champions in the history of chess until the year 2000, when Vladimir Kramnik seized the title from Kasparov.
Far more extensive, however, are the world championships matches themselves. According to ChessBase, the last clash between Kasparov and Karpov was world match no. 35. Afterwards, there were three Kasparov duels which played outside the reign of FIDE. A second database is dedicated to the six FIDE world championships from 1993 until 2002