The Ancient Druids did not keep records of their Numbers and Julius Caesar did not do a Census of them before Massacring them and Destroying their Groves and during the Dark Ages and the Burning Times, there were never any Censuses done.
I don’t think Julius Caesar massacred the druids to be honest, in fact there were tribes allied with Rome and they, presumably, would have had their druids who remained unscathed. The Romans got tough on the druids under August, Tiberius and Claudius much later on, and they never really conquered Scotland or reached Ireland at all. Julius Caesar’s forays into Britain were not really conquests and he struck up a couple of agreements and deals with locals kings in the southeastern areas. There were also Celtic tribes in Britain that never picked a fight with Rome and we might assume that they kept their belief systems and social structures relatively intact.
During the witch trials, I don’t think anyone was ever burned at the stake for being a druid. It’s important to note that heretics, i.e. Christians, were burnt at the stake, whereas witches were hanged, so again we might presume, from an historical point of view, that no druids proper were persecuted during those times, the main reason being that there weren’t really any druids around by then. Interestingly, the witch trials were relatively scarce in “Celtic” areas and few people were persecuted in comparison with other parts of Europe.
In order to estimate how many druids there might have been at any one time, and it will only every be a rough estimate, it might be better to think about how many tribes there were, each with a chieftain and, as we often see in the mythological materials, each one with his own druid. So, if there were thirty Celtic tribes in an area, each one with a king and a druid, that would be equal to thirty “official” druids – as a minimum. It’s not much to work on, but it does suggest that there may have been relatively few druids compared to other roles in these societies.