I once made a theory, or perhaps rather a casual remark, that I believe every TOP LEADER will be in charge of several conquerors. And depending on how many conquerors that guy is in charge of we can gauge that leaders level.
Imu - 5 (Warcury, Venus, Mars, Jupeter, Saturn)
Xebec - 3 (WB, Big Mom, Kaido)
Roger - 2+1 (Rayleigh, Gaban + Oden for a short while)
Luffy - 2 (Zoro, Sanji)
WB - 1+1 - (Oden, Ace, they never coexisted)
Kaido, Big Mom - 1 (Yamato, Katakuri)
Dofamingo, Don Chinjao, Boa Hancock, Kidd + all CoC-followers - 0
Dragon, Sengoku, Akainu, BB, Joyboy, Shanks and so on can all be plotted in as we get more information. But as a general system I think this will be rather accurate as for how much of a conqueror a certain Captain/Leader is. And as evident Imu is as of now top dog by some margin, at least 5 confirmed (and lets see about Garling for example).
Xebec who died prematurely at the hands of Imu and Roger to sickness both reached 3 at one point. By this hierarchy Luffy has now surpassed WB who had 2 but never at once, a feat that still puts him above Kaido and Big Mom who only ever had one. And at the bottom those who had 0 or who choose to follow another CoC-wielder such as Zoro following Luffy or Rayleigh following Roger.
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Sanjis CoC is thus expected. Yamato if she joins would essentially mean Luffy joins Xebec and Rogers table of 3 CoC-users. Still need Loki + 1 more to reach 5 and equal Imu.
We can probably expect Shanks to have at least 2-3 (Beckmann, Lucky Roux, Yasopp) and Dragon to have 2-3 (Sabo, Ivankov, perhaps retconned Kuma or undercover RA like Fujitora or Kuzan)