* Language is constantly evolving: New words are coined all the time, and old words fall out of use.
* No central authority: There's no official body that tracks and records every single word in the English language.
* Definitions are subjective: What constitutes a "word" is debatable. Do we count proper nouns? Slang terms? Technical jargon?
While it's fun to think about a "millionth word," it's more realistic to acknowledge that the English language is a dynamic and ever-changing system.