Conflicting estimates of total Jedi numbers prior to Order 66 appear across multiple "Star Wars" novels, games, and television shows, but in the wake of Disney's acquisition of the franchise, many are no longer canon. That means the clearest, most canonical figure we can rely on comes from a Season 1 episode of "Star Wars Rebels."
In the episode "Path of the Jedi," Kanan Jarrus, a Jedi who survived Order 66, tells his apprentice Ezra Bridger that during the halcyon days of the Republic, there had been a sum total of about 10,000 Jedi Knights. Given that across all of "Star Wars" media we've seen only a few important Jedi who survived Order 66, it's safe to say that just under 10,000 were killed as a result of the regime. In a galaxy as densely populated as it is in "Star Wars," that isn't a lot of manpower, which helps make sense of the fact that clone troopers were able to slaughter powerful Jedi en masse. They overpowered their force-wielding commanders with sheer numbers.
However, Kanan's use of the term "Jedi Knights" complicates the matter. Jedi ascend ranks from Padawan to Knight, and a select few progress to become Masters. Anakin Skywalker was famously allowed to sit on the Jedi Council without being granted the rank of Master, for example. So it is possible that Kanan was only accounting for Knights, but not Padawans, Masters, or other niche titles Jedi could acquire. Even so, Knights were the most common Jedi and comprised the bulk of the Order, meaning that even if Kanan's figure of 10,000 only includes them, it wouldn't be far off from the actual figure.
