You're conflating two topics.

1. How many people using a word in an existing language makes that word a word from a descriptivist context?
2. How many people does it take to make a conlang?

The answer to the second is, of course, one, but that's a very different question than the first one. The first question doesn't have a definitive answer. The more people that use some new word the stronger the claim is that it is in fact a word is, but there's never a moment when you can definitively say everyone agrees that it is a new word. (If you could then we wouldn't need to ask the question at all lol.)