He had three apprentices when we first met him: Goji, Quai, and Reishi, in increasing order of seniority. Reishi has been an apprentice for decades, and will likely be an apprentice and bottle-washer forever, being little suited for the more complex tasks; he is Master Hsi's nephew. Quai is the most ambitious, and burns to be a master alchemist on her own, but Master Hsi is a strict teacher and has not granted permission yet. Goji is young, and has less drive but possibly more talent than Quai.
The party returned to Master Hsi for another Tincture of Pure Respite during Testing, One Two Three. In return Kuan-Xi has made a water gate for him to the Dragon's Throne, the location of which was resolved in Tying Loose Ends.