It might be roughly 800,000,000 million people. Basiclly how i got this number is, i took the population density of the hight of rome and applied it to roshars size which apparantly is about 40,000,000 square kilometers. I cut some of it out because of the unclaimed hills and the frostlands but i think it sounds reasonable. It might even be a low estimate since roshar can relay on soulcasters for food and not just on farming