Clapham Junction (which is essentially 3 different stations joined together and sharing the same name & platform sequence) only has 16 platforms (Numbered 2-17).
Edinburgh AFAIAA doesn't have 21 platforms, they are numbered up to 21, but they aren't all in use, with some different numbers being the same platform.
The most is Waterloo. 1-19 on the domestic side. International is another 5. Waterloo East has A to D, and the Waterloo & City Line platforms are numbered 25 and 26. Then the "real" Underground platforms add another 6, for 36.