Roughly 275 hundred years ago, 40,000 terran were sent to the korprulu sector. Within the first 100 years, their would be a period of instability. Factoring in both the alien environment with the increased life expectancy, it would be logical that death rate is .8% or so, roughly what we have today, the hostile enviroment counterbalancing vastly improved med tech. Birth would be high, lets put the average at 5, something comparable to developing nations today. (it is higher, because the terran population can afford to sustain this rapid growth because of vastly improved tech and a planet of untapped resources). Lets assume five generations are created within the century, something else comparable to developing nations.
After a century, we get 125,000,000 people, factor in the death rate, and we get roughly 85 million left standing.
The next 175 years, the population should stabilize to say, 3 births, but the death rate would go down to say, .7%, kept up only because of wars, and 25 years per generation. All reasonable estimates.
That gives a whopping 65 billion people, and after factoring death rates, leaves us with 19 billion.
Factors that limit population size, like land available, and resources, are less problematic due to colonial expansion into several solar systems, terraforming, and tech like sustainable cold fusion.
Makes sense.
edit2: Yes, I know that most numbers I pulled are just estimates based on real world data, not blizzlore, all save the timeframe. A single +1/-1 in almost any field would alter the current population exponentially. But my numbers all make sense, and show that the terran population could be easily in the range of dozen+ billion. Even so, keeping the numbers with reasonable ranged, we could get populations between 5 billion ish up to 40-50 billion.
100,000 million...
makes no sense what so ever, unless infant mortality is ridiculously high. As in 50%+. And life expectancy was in the mid 40s. Which alone is just ridiculous.