Well, since no one's guessing numbers, i'll pull out my educated guess:
Based on the fact that Capella was a major system, yet it only had 200 million residents (equivalent to US population), and that Capella is fairly recently colonized Vega, Delta Serpentis, Beta Aquilaie, and Antares should probably have populations similar to Capella. As well Cygnus Prime in the Deneb system has something like 100,000 colonists, which is less than the population of Ottawa (the tiny capital city of Canada). since they were colonized much earlier, i'll say that each system has about 500 million to 1 billion residents
Dubhe: 0-3,000; remote system, newly discovered so there's little chance of any major colonization
Alphard:0-3,000; remote system, newly discovered so there's little chance of any major colonization
Wolf 359: 10,000-100,000; It is stated that the GTVA has a fleet there, so there should ideally be something of worth to protect
Barnard's Star: 0-10,000; another remote and newly discovered system, i doubt there could be any major settlements
Luyten 726-BA: 100,000-400,000; There was obviously enough people here to form a Terran powerbloc, so i douby that the population is below 100,000
Laramis: approx. 250,000; since Luyten is probably decently populated, and Laramis was discovered first, not to mention the RNI shipyards, this system probably has a population matching or superior to Luyten's
Ross 128: 0-150,000; In FS1, it was stated that Ross 128 was an outlying colony outpost, so there was probably no large concentration of population
Delta Serpentis: 500 million-1 billion; this being the system first colonized by humans, it probably has a large population, much larger than Capella, which was colonized after the Great War
Regulus: 200,000-10 million; the system's proximity to Polaris and Sirius, who should both have elevated populations, leads me to think that Regulus must have a decent population in the millions
Polaris: 15-100 million; Admiral Bosch obviously saw something in this system, so I doubt he would choose a barren system for his symbolic capital. It being Polaris, I think it should have a pretty high population
Epsilon Pegasi: 1-50 million; Considering Enif's proximity to Capella and Polaris, and its seemingly high importance in FS2, I believe that it should have a population in the millions at the very least. No possible way it could be under a million or else the slaughter of 80,000 people would have been an economic disaster
Mirfak: 0-10,000; This system seems to be of little importance, but we have limited info on it so I cant say much
Adhara:0-10,000; Same as Mirfak
Procyon A: 0-5,000; Same as Mirfak and Adhara
Capella: formerly 200 million; from Cannon facts we know this number, and let us remember that prior to FS2, we never heard of Capella, which probably means it became colonized after the T-V war, which would mean that 200 million moved in during 32 years.
Altair: 500,000-10 million; It was recently discovered in FS2, but it became home to some big shipyards, and then it became adjacent to the new VAsudan Capital. Altair probably became the destination for
of Vasudans fleeing the Lucifer.
Aldebarran: 400 million-2 billion; As the new capital for the VAsudan gov. it should have hundreds of millions of Vasudans living there.
Alpha Centuri: 300-800 million; With three Jumpnodes, and its proximity to Vasuda and Aldeberran, this system was probably colonized early in the Vasudan's space-age, meaning theres gotta be a lot of them
Deneb: 50 million-100 million; Deneb seemed to be of relatively great importance in both FS games, and it being adjacent to Vasuda leads me to beleive that theres a large concentration of population. But the fact that theres only 100,000 ppl on a planet may shatter my assumption
Beta Aquilae: approx 2 billion; New GTVA capital, nuff said
Sirius: 500,000-40 million; It was a colony in FS1, and it probably swelled in population after the Great War and during the Reconstruction
Vega : 500 million-2 billion; This system was of critical importance in FS1 and it sounds like it is a major centre in FS2, so it probably has a huge population
Antares: 100 million-1 billion; Same as Vega, and the fact that the Antares FEderation actually existed as a powerful bloc
Remember: These are all just assumptions
