How is it ever proven or demonstrated as to the final number of secret Pokemon cards in a series? These are the cards that are numbered higher than the set number e.g. 194/192 and tend to be holos, rainbow holos etc. The cards released on the official website only number up to the "official" amount.
The full set list including secrets is published in a booklet that comes with the Elite Trainer Box for each major set. Beyond that, major retailers and streamers often unbox several cases during the day or two prior to the release and determine every card that way.
Finally, it is known well before the Worldwide (English/European) release what is possible due to the Japanese released sets always coming earlier. While it’s possible some secret rates from a Japanese set don’t make it into the corresponding worldwide set, in general most or all potential ones do.
Pokémon Trading Card Game Online, official simulator of the card game, has all cards available, including all secret rares. You can access all cards e.g. through Collection mode with checkbox Show Not Owned checked.