I'm curious if the average adult knows more words in their language (excluding proper nouns), or more proper nouns? At first I'm inclined to think the former, but then I think of all the names I know: names of people, places/streets, institutions/companies, brands, titles of books/movies, etc. I know this is a difficult question but I'm curious how to estimate it.
A 2016 study suggests that your average 20-year-old knows over 42,000 lemmata ("basic" words, like run, as opposed to running and runs). Multiply that by three if you want to include "non-basic" words: as a rule of thumb, English nouns have two forms, verbs four, adjectives three.
While there are a lot of proper nouns out there in the world, it's unlikely any individual will know more than 126,000 of them.