Board Thread:Questions and Answers/@comment-9826910-20130618064245/@comment-8986886-20130722230427

At least Yamamoto and two more Captains must have seen Isshin's Shikai release, during his 'Captain Proficiency Test' and since Mayuri Kurotsuchi knows the Bankai releases of all Shinigami who achieved it, he should also know Isshin's Shikai.

That makes at least three, probably four Captains who knew the name of Isshin's Zanpakuto; depending on if Mayuri was one of the Captains overseeing Isshin's Captain's exam. Then Aizen, Gin and Kaname witnessed Isshin fighting White, using his Shikai.