"Masamune Juttetsu" can be thought of in the same sense as the phrase "God created the world in 7 days" – even most modern-day Christians believe it to be, at best, an allegorical phrasing or a mnemonic for teaching scripture. Here's where, to my knowledge, the current consensus stands regarding which Juttetsu may actually have been taught directly by Masamune:
YES (with an asterisk)
Norishige - As we now know, he was less a pupil/disciple of Masamune than a peer, yet nonetheless he would have learned directly from Masamune in the same way any artisans in the same workshop will generally trade techniques and provide assistance
Go - The same as above; the large amount of crossover between Go's work and both Norishige and Masamune suggests he was an active presence in the same workshop (as do the records of Norishige mentoring Go's son, Tametsugu, after Go's untimely passing)
MAYBE
Shizu Saburo Kaneuji - The abrupt change in style — combined with many of his works being so similar to Masamune's that the two often swap attributions — suggests there's truth to the tales of him being, if not a direct student, then at least in the same location and learning from someone with direct instruction on Soshu smithing
Samonji - Again, an abrupt change in style with a clear delineation between his more provincial style and his excellent Soshu work. Given just how different the techniques and materials are between the two styles, it's likely that he was either a direct student, or had close association with someone who was
Hasebe Kunishige - Opinions currently divided on whether Kunishige learned in Kamakura, or sent someone there to pick up some techniques and bring them back. There may in fact have been several smiths with that name working concurrently, one of whom made the trip to Kamakura to get some tips or scope out the competition
UNLIKELY
Rai Kunitsugu - While his work does show a more dynamic and nie-rich hamon than his predecessors and contemporaries, I hesistate to actually call it Soshu or even a Soshu hybrid rather than simply an evolution of the mainline Rai style. We see other Rai works with the same qualities, even from his father, suggesting the Rai group were able to come up with these innovations on their own as a response to changing tastes
Kinju - Similarly to Rai Kunitsugu, he was likely influenced by changing tastes among the clientele in the region and as a result made "Soshu-like" work
Sekishu Naotsuna - The dates don't add up, and there's evidence of other smiths in the region around the same time and even a little earlier experimenting with Soshu-style innovations. Most likely Naotsuna was simply the latest and most successful among the smiths in that region to blend Soshu technique into his own workmanship
Osafune Chogi and Kanemitsu - Again, the dates don't add up. They likely learned Soshu techniques from Nagashige, who is now believed to be their older brother or uncle and who currently has the oldest dated and verified Soden-style blade (the Kinoe-Inu Nagashige, dated 1334)
Kongobyoe Moritaka - Was, in some older texts, sometimes considered to be one of the Juttetsu despite his work being further from Soshu than any of the current Juttetsu; this further supports the idea that the Juttetsu are more of a mnemonic/allegorical construction than a literal one
In the same vein, the best smith we know who most definitely was Masamune's pupil was Soshu Sadamune, yet he is not included among the Juttetsu (presumably by dint of being Masamune's heir; a student of nihonto would not need a mnemonic device to remember him among the luminary adoptees of the Soshu tradition).