What it says on the tin. How many terrans, protoss, zerg exist? Surely billions of terrans exist in the Koprulu sector at least. According to the sizes of the zerg broods in the SC1 manual, there are(or were) only about 12,171,000 zerg.
We aren’t really shown how fast zerg or protoss or even terrans grow up. To think that there have not been additional adult generations since the carnage of the Brood war(if terrans age as fast as humans do)- that would mean that surely at least a hundred billion would exist.
Or perhaps terrans have cloning technology.
Protoss seem to not procreate much. None of the major characters are below 200 years old unless I missed a spot somewhere.
The zerg, well, I can’t think of anywhere that mentions how fast it takes to birth a zerg.