According to Official Guinness Records,
The most abundant species of monkey for which a total population size estimate has been produced by researchers is the gelada Theropithecus gelada. Native to Ethiopia and related to the baboons but vegetarian in diet, according to the IUCN this species has a current estimated total population of approximately 200,000 individuals. Overall densities based upon surveys conducted in a variety of different locations vary between 15 and 60 individuals per square km, but densities of animals within home ranges frequently exceed 70 individuals per square km.As stated by the IUCN: "Geladas are widespread throughout their present range but, partly as a result of the droughts affecting the Horn of Africa in the 1980s, they are not as abundant as in the 1970s when an aerial survey of the central Ethiopian highlands yielded an estimate of 440,000 for the total population. An alternative estimate based on known ground densities and the total area of gorge face on the plateau yielded a figure of 880,000. C. Hunter (pers. comm.) considers that these estimates may now be too high, and that a better guesstimate would be approx. 200,000 animals".
For a complete list of 2015 records, please visit 2015 Guinness Records in Ethiopia.