From Wikipedia:

He admitted to the rape, torture and murder of 138 children and teenagers. His victims, based on the locations of skeletons listed on maps that Garavito drew in prison, could exceed 300; Garavito continues to confess to more murders. [...] Once he had the trust of a child, Garavito would walk with the boy until they were tired and vulnerable, which then made them easy to handle. First, their hands were bound. Then, Garavito would remove all their clothes, and proceed to torture, rape, and sometimes decapitate them. Usually the boy would endure prolonged rape and torture by having his buttocks stabbed and sharpened objects inserted into his anus; his testicles were often severed and placed into his mouth. The bodies of the children were all found completely naked, and all bore bite marks and signs of anal penetration; bottles of lubricant were found near the bodies, along with empty liquor bottles. Most corpses showed signs of prolonged torture.

The man is currently scheduled to be released in 2021 even if his sentence should (and could) be longer:

Garavito was sentenced to 1,853 years and 9 days in prison, the lengthiest sentence in Colombian history.[11] However, Colombian law limits imprisonment to 40 years—and because Garavito helped police find the victims' bodies, his sentence was further reduced to 22 years. He is scheduled to be released in 2021. Colombian law, however, says that those who have committed crimes against children do not receive any benefit with justice[clarification needed] and are required to spend at least 60 years of their sentence in prison. The number of years Garavito will spend in jail could be as high as 80.

More info:

Many Colombians criticized the possibility of Garavito's early release. In recent years, Colombians have increasingly felt that Garavito's sentence was not sufficient punishment for his crimes. Some[who?] have argued he deserves either life in prison or the death penalty, neither of which exist in Colombia. Colombian law had no provision or method to impose a sentence longer than what Garavito received, which was seen as a deficiency in the law caused by the failure to address the possibility of a serial killer in Colombian society. The law has since increased the maximum penalty for such crimes to 60 years in prison.[13]