(quoting @rich)
I had seen similar numbers in a few articles by polyglots I’ve been reading lately.
However, they underlined the importance of focusing on high-frequency words when learning a language. According to studies if you choose wisely - rather than random vocablulary (can’t remember sources, but had saved the numbers):
10 words give you 24% coverage (of words you’ll hear/read in average communication/text and will allow you to understand enough of it)
100 => 49%
1000 => 74%
2000 => 81 %
4000 => 87 %
20000=>98%
I had found an on-line unique word counter, that’s not reliable for a serious study - it counts as unique mutations, plurals, anything with an apostrophe, names, mistakes in copy and paste or spelling etc - but I think it’s good enough to play with! 
The average Sgwrs contains an average of 900-1000 unique words
In the first 15 (if you copy and paste them all together) there were 5694 unique words out of 52113 total
As of today (from 1 to 22) 6807 unique words out of 70997 total
So a whole lot of words are just the same, in different combinations and accents! 
p.s.
in SSiW Level 1 to 3 vocabulary lists it was about 750-800 unique words
p.p.s. I just realized I posted it in a different thread, but maybe it’s even better cause it’s a more generic topic, and not specific to number 21…