Chipworks has published an update to their Apple Watch tearsdown that takes a look inside of the S1:
Inside the S1 SiP alone we have cataloged more than 30 components. Now, some of those components contain multiple die; the package-on-package (PoP) assembly, for example, contains the new Apple processor and the DRAM die. The NFC solution also contains the secure element as well as the NXP NFC controller and radio. So there are 30 individual components, and at least 30 pieces of silicon, all in a package that is only 26 mm x 28 mm. That is quite an accomplishment.