I can guess that the number of missing incumbents is around a couple of thousands. Many of them ruled countries of which the average westerner never heard. :-)
SB> Yeah, it would be a shame if *notable* former rulers were missing...
Perhaphs a guy who ruled a 19th century Europan country (population: ~5 million) for about 10 years? :-)
http://en.wikipedia.org/wiki/Alexandru_II_Ghica (Wallachia; 1834-1842 and 1856-1858)