Estimates of the number of Christians in the Roman Empire at the time of Constantine's conversion vary widely. Some scholars suggest that Christians may have made up as much as 10% of the population, while others put the figure closer to 5%. This would mean that there were between 5 and 10 million Christians in the empire at the time.
It is important to note that these are just estimates, and there is no way to know for sure how many Christians there were in the Roman Empire. However, it is clear that Christianity was a significant religion in the empire, and that it was growing rapidly at the time of Constantine's conversion.