I am wondering if there is any actual reputable scholarly sources for this. Mostly I ask because when I try to google it, I get only extreme right-wing websites including some with overt nazi tendencies. And on the wikipedia site for this subject, there are plenty of people in the talk page noting how ridiculously biased and skewed the numbers presented are. So I am honestly just wondering if any professional scholars have a decent estimate for this. I put 'killed' in quotes because obviously there is a need to make sure that any death total was truly intentional by that government, and I put 'Communist' in quotes since I know it is a matter of some political dispute as to whether or not a given country's government could truly accurately be called that, even if it is commonly referred to as such by laypeople. Thanks in advance.
Also I want to note up front my intention is not to really "legitimize" attempts to implement Communist ideals, but simply because I feel a great skepticism about some of the more shall we say ostentatious numbers which seem to have become quite popularized.
A really awesome thing would be to try to track like killings by year as well, so we ccould say "the peak of killings was during X situation" and so forth. And maybe give some context to certain killing spikes which could help with understanding why they happened.
Stalin: Recently-opened sources from Eastern Europe and Asia have given us a lot more information on the costs of Stalin's rule. Yale Historian Timothy Snyder gives a rundown of the figures in this article, but I highly recommend reading the entirety of his book Bloodlands for a fully detailed analysis of Stalinist repressions. Tl;dr Stalin's rule (through executions, famines, Gulags, deportations) caused a maximum of 9 million deaths. The famous Robert Conquest, the recently-diseased historian, scaled down his earlier estimate of the # of victims also. However, Snyder emphasizes that just because Stalin killed fewer than originally believed, doesn't mean he is now 'rehabilitated'. In fact, before WWII Stalin had killed far more people via ethnic targeting than Hitler (e.g. through the NKVD killings of ethnic Poles). This shows how Stalin was not hesitant to single out entire ethnicities when they advanced his political goals.
Mao: Yang Jisheng, a Chinese historian whose father perished in the Great Leap Forward Famine, gained access to Chinese Communist sources for his book Tombstone. He outlines in it how Mao and other Party members were aware of the Famine and that it was a largely man-made disaster. By looking at both previous estimates and Chinese govt records, Jisheng estimates 36 million died from 1959-1962 (due to both famine and violent state repression).
I know less about modern estimates of the deaths caused by Mao's other policies and by other Communist regimes such as Pol Pot.
While I haven't got specific numbers handy I highly recommend 'The Gulag Archipelago' by Solzhenitsyn. It's a comprehensive if very personal timeline and historical narrative of the communist regime in Russia. Solzhenitsyn lists more than 50 methods of 'interrogation' for political prisoners. He describes the 10-year labour camp sentences given to teenagers who stole ears of wheat from fields. It's a very interesting and well-written, if depressing, read.