Not to be pedantic, but it really depends on how you measure it.

Obviously taking a political opponent up against a brick wall and shooting him counts. But how about someone who is put into a gulag and dies? How about if someone spends a year in Siberia, is released, and then dies from something he caught while in prison? That was actually a policy for a while, to release sick prisoners so they could die at home. How you measure will radically change the number of how many political dissidents were killed.

Same thing for stuff like the Holodomor (hunger extermination). There was a real famine in 1932-1933. And yet somehow most of the starvation happened in Ukraine especially in areas where the communists had low support. It's an open question how many people died, estimates vary from below 2 million to more than 12 million, with 5 million being a very rough consensus number. Regardless of Stalin's policies, some people were going to die. There was a real lack of food on the ground. How much of the death can be attributed to the famine, and how much blame should go to Stalin? He clearly exacerbated the situation and made it worse. He also largely chose who would die. So if we assume (just to pull some numbers out of my ass) that 6 million died. And if we assume that 2 million would have died regardless, 2 million came from incompetence, and 2 million came from malfeasance. How do we divide the numbers up? Does Stalin get credit for 2, 4, or all 6 million? And that's assuming our 6 million is correct. It could easily be twice that, or half that.

How much "credit" does he get for WWII? Clearly most of that ledger goes to Hitler. But Stalin added plenty via purges and mismanagement. In the end the Soviet way of war was successful, but it was terribly bloody both on troops and civilians. The Soviets were committing massacres against Poles for example as early as September 1939. And they continued doing so against a variety of peoples until the war's end. I've seen estimates that over a million German soldiers died in Soviet prisoner of war camps, although official Soviet numbers are closer to 360,000. It's likely that over a million German civilians were killed by advancing Soviet troops. And that not even touching on the uncounted millions of Soviet citizens that died during scorched earth and reprisals.

So in the end we likely have to shrug our shoulders and say, "I don't know." Easily tens of million of people died unnaturally early due to decisions made by Stalin. Did it reach 50 million? I'd say likely not. My educated guess is that it's likely closer to 20 million, which is also the number Matthew White came up with. He's probably studied the question as closely as anyone, but he's not a professional historian, so his conclusions should be taken with some skepticism. But given how loosey-goosey all the death figures of the time and place are, all of them should be taken with more than a grain of salt.