There's a thread about the same topic:https://columbinemassacre.forumotion.com/t5463-serial-mass-murderers
But I have some examples of serial mass murderers!
Anatoly Onoprienko killed a number of families in robberies during the 1980s and the 1990s.
Luiz Gonzaga Pereira dos Santos killed a family of seven in 1979. He was also credited with four other murders and other crimes in the city of Pernambuco and Paraíba, though i am not sure if the four deaths all happened at once, since his Wikipedia article doesn't say anything about those crimes.
Ramiro Matildes Siqueira who according to his Wikipedia article some entires families were murdered in his crimes. In all he was accused of 54 crimes. I don't know how much of his Wikipedia claims are true, but apparently some editor added some sources but removed later, due to being worried by copyright.
Turabaz killed 68 people. He was mentioned on a article about Harphul Singh, in 1930. The mention about him was very vague, as it just said "The government has offered a reward of land and cash for the capture of the killer (Harphul) whose reputation is surpassed only by that of Turabaz who killed 68 people before a police bullet ended his life." I assume that he was a serial mass murderer, and bandit.
Harphul Singh (mentioned above) who killed 11 people in Tohana, British Raj on July 23, 1930, did also killed three people in 1928, and murdered two more a month later.
In the early 20th century an Ifugao convict killed seven other inmates at a prison in the Mountain Province, Philippines. He was arrested, was later convicted of the murders, sentenced, imprisoned, where he remained inconspicuous, until he ran amok and "killed several other persons"
George Jefferson Hassell killed his wife and eight children in 1926. He had previously killed a family of four in 1917. Thogh the crime isn't mentioned on Wikipedia, you can see a news article about it here.
Manoel dos Santos killed 1 to 2 persons and wounded 9 to 11 others in São Luis, Maranhão, Brazil in June 1946. He declared that he had already killed nine people before he was lynched. He belonged to a family of assassins of João Pessoa.