The Knightly Orders didn't have a fixed number of men, they expanded and decreased based on their popularity. The Templar and other orders had large numbers of Knights and controlled as much territory and wealth as a kingdom, but I'm not sure the exact numbers.
Some of the european history books should have access to that information, as the knight orders obviously themselves kept track of the information. Because the Templar order was later banned by the King of France, its possible that no actual records remain of how big the Templars once were.