Someone seems to have taken the highest estimate of the total number of people allegedly killed by Mao & the total number by "Stalin" & combined them. That would make 124 million. It doesn't count those attributed to "Lenin", Pol Pot or Castro.
It assumes that the above practiced the communism proposed by d'Hupay in his 1777 book "Projet de communaute philosophe (Project for a Philosophical Community)" &, later, by Marx & Engels in "The Communist Manifesto (1848)". However, all in fact practiced "Leninism" or "Bolshevism", the radical variant devised by Ulyanov ("Lenin") which was forced upon a poorly-educated agrarian society, rather than occurring by social revolution in an educated, industrialised society as proposed by M& E.