A "lack of info" is a good way of putting it. While Belgian estimates indicate something around 10 million inhabitants c.1930 (the country's first real census wasn't until 1984), there just aren't meaningful population data for the period of Leopold's personal rule (1885-1908), still less for any time previously, and hence no sound basis for an estimate of total excess mortality inflicted under his exceptionally brutal regime of exploitation.

I haven't encountered an estimate of up to 15 million deaths before, but the more common 10 million appears to derive from late 19th-century European guesstimates of a population of 20 million, at a time when there was simply no way of knowing. Overestimation of African and other non-European populations was not uncommon at this time, the same figure circulating for northern Nigeria when later counts found fewer than half as many. In the case of Congo and other territories it seems likely that a part was played by speculators in the "Free State" venture eager to "talk up" the capacity of the territory on offer (Zelinsky notes a frankly ludicrous claim of 43m by Leopold's enabler Stanley for an "upper Congo section", whatever that might be).

It's important to recall that Leopold's system of free rein to European concessionaries supported by extreme brute force was above all a system of labour expropriation. The area was never considered an area of intensive European settlement and was unfavourable to it, and the regime's grotesque abuses were geared primarily into forcing Africans to produce rubber and other produce for the king and his business partners: the purpose was vicious exploitation of human beings rather than their eradication which would have left the rulers and their accomplices without an income.

That there were severe demographic impacts seems beyond question. Local resistance was crushed mercilessly, Africans were subjected to daeth or mutilation for failing to meet their rubber or ivory quota or just to balance the spent ammunition accounts; whole villages fled to the forest or adjacent (and themselves oppressively-governed) French or German territories to escape the insatiable labour conscription regime, and it is said that women avoided bearing children who would become fodder for the abusers: the forest belt long remained a zone of exceptionally low fertility here and in neighboring concessionary-exploited colonies, though the origins of the anomaly remain obscure.

But it remains impossible to put a reliable figure on the deaths. A toll in the low millions is plausible, but there is no reliable evidence for any particular number: Caldwell indicates ten million excess deaths in the whole of Africa in 1880-1920, a period covering colonial conquest and the deadly post-1918 influenza episode for which more recent findings may indicate a higher total. That doesn't make Congo's experience under the Belgian king any less horrific an example of exploitation at its most depraved, but this was to be a hell for the living rather than an empty land.

While accounts of the regime's barbarities are plentiful, discussion of the actual demographic impact is scarce and necessarily tentative: for an overview, see JC Caldwell, Social repercussions of colonial rule: demographic aspects, in General history of Africa v. 8, Unesco 1985; and for an early critique of population figures Glenn Trewartha & Wilbur Zelinsky, The population geography of Belgian Africa, Annals of the Association of American Geographers 44:2 (Jun 1954).