Originally Posted by
allset2travel
In the Tokyo Station, the Tokaido-Sanyo Shinkensan platforms are #21-24...
No.
The OP can simply look the platform up on Hyperdia. The Hikari #505 that he was looking at leaves Tokyo from platform #17. The other Nozomis and Hikaris all use #14-19.
OP: Hyperdia shows "DepartureTime/Platform" - Tokyo(8:33) 17. The 17 is the platform.