I was once asked how many people studied a koryu historically. One that I have looked into was the Shindo Muso Ryu Jodo.
Nagatomi Koshiro Hisatomo (1717-1772), the seventh headmaster of one of the three lines extant at that time had 300 students
During the Bakumatsu, or the mid-nineteenth century, the three lines of Muso-ryu were very active. There were eighteen menkyo holders in the Haruyoshi line, fifteen in the Jigyo and nine in the “true path”.
Shiraishi Hanjiro (1842-1927) was one of six people eventually awarded a joint densho between the first two of those lines, the third having disappeared by then. Shiraishi was originally a student of Hirano Kichizo and Sada Teisuke of Haruyoshi. He later received mokuroku from Okuma Shinpachi of the Jigyo line. His training before receiving the joint menkyo was from Yoshimura Hanjiro. Shiriashi was the sole instructor of Shindo Muso Ryu Jodo by the end of the Meiji.
Shimizu Takaji began training with Shiriashi in 1913, at the age of 17. In 1918, at age 23, he received his mokuroku (scrolls of transmission) and two years after that his menkyo certificate. In 1939 during the Pacific War Shimizu went to Manchuria to teach jo and one author states that he eventually taught jo to 1,500,000 people.
So the answer would be… it varies with the political situation (Jodo was big in Fukuoka where it originated, until the Han system was disbanded, and then the numbers collapsed), war and peace (the imperial army studied Jodo so all the recruits were “students” of koryu), the instructor (popular instructor in a big city with good connections and publicity means lots of students), the organization (Shimizu joined jodo to the ZNKR which brought in a lot of students) and the definition of “koryu” (if ZNKR jodo is disallowed, jodo is a lot smaller than it “is”).
Any organization, koryu, modern, fraternal or what have you, tends to fragment once it gets to a certain size. It’s the natural way of things, only an extremely good leader can keep things together for long after a critical mass has been reached.
The other way fragmention is resisted is through an external (extra-lineage?) force, such as you see in the FIK/ZNKR, which can (and has) actually put national Kendo groups back together after a split. The “glue”? … if you’re “out” from the FIK you don’t send competitors to the world kendo championships. There are other kendo organizations but they are dwarfed by the FIK simply because of the mechanics of the World Championships.
Few such forces exist in the koryu outside of “legitimacy” which is why it’s so tediously argued about. Papers, signifying objects that are passed along, grading systems, or just plain old publicity can all be used in the arguments of legitimacy. One “external force” this legitimacy may be predicated upon, is the existence of an outside sanctioning or pseudo-sanctioning body which may declare one group or another “legitmate”. Barring membership in these organizations, the groups are left to rely on such things as…. say…. the internet to argue their legitimacy.