This assumes you only know your native language. For example japanese, Each book of japanese from zero books teach 800 words. There are 4 books. That makes 3200 words. 3200 words is not far from knowing all that is needed.
This article I found serves the purpose of this post.
i belive an adult person knows more than that !
my native lang is arabic 2ndry lang is eng , yet i know a little bit of russin germen france japanese and chinese
i think it can reach up to millions of words
It’s true that each langauge contains millions, but the people only speak thousands of those words. Most of the words are not in use.
It would be impossible for a person to learn one million words, he’s life span is too short. It’s impossible to count from 1 to 1 million in a life time. One second apart.
maybe !!! it might take you a day or 2 to count from 1 to mill
I pulled out my calculator and computed 8 hour days. I came up with 34.72 days.
This means that either the teachers in my 3rd grade school lied, or they are just stupid. They told the entire class that we could not reach there in a life time. This is not the first time they have given us trash.
i really didnt understand !!!
the book is probebly incorect, havent thy said that iducation around the world is brokin ? thats probebly why ?
But kindly remember that both Japanese and Chinese are ideographic languages: words are represented by symbols that must be recognized by sight, and which do not correspond to the pronunciation of that word.
Whereas most people, when “reading the newspaper,” upon encountering an unfamiliar word, could at least take a guess at pronouncing it, a Japanese or Chinese reader might well have no idea – and, no clues. (S)He would have no choice but to skip to the next symbol that (s)he does recognize, and to infer the intervening meaning from context … possibly being unable to do so.
Always question your teachers 
just not for the sake of it.
Also from those millions of words, a lot of them can be derived if you know the basics (think of compound words, prefixes, suffixes, adjectives&adverbs etc…)
Anyways, I can only recommend learning a second language. It is not only a nice way to extend your cultural horizon but can also be a proper solution to channel your procrastination.
I started learning another language just for fun some years ago and now whenever I want to avoid doing stuff that I really have to do, instead of sharpening pencils or browsing the internet I just learn some new words I will probably never use… Which feels more of an accompishment than getting reddit karma or facebook likes or having ordered my crayons by color…
I also recommend something like duolingo or a similar gamified app to get your feet wet in a certain language. I just started with danish and it is a lot of fun. I can do it on the subway, waiting room, bathtub…
Well, I can tell you first-hand that “language ability” is a recessive gene. … 
My mother can fluently speak German, French, and English. (And my Dad gets by …) Whereas, I spent two full years in Puerto Rico and still can’t order a pizza in Spanish!
(Oh, I tried very hard, and so I winced … good-naturedly, of course … when the proprietors of the local shop, fully understanding my desire to speak the language, patiently explained that I had just ordered a tasty pie with an automobile muffler as a topping. Or something like that …)
just get a good knock on your head 
My book called, ‘japanese from zero’ will arrive today. Maybe in about 1 hour. I’ve decided to learn japanese. I don’t know what the secret desire is, but for some reason I really want to learn japanese. Next after japanese, is swahili.
Whatever might be the argument, learning a new language does help in improving the vocab significantly. However, is it true that languages like Japanese and China are pictorial and thus comparitively more hard to learn?