I (native American English speaker, college prep school educated) had 5 words that I thought I knew, but still got wrong:

obsequious

laconic

sanguine

quotidian

enervate

On the other hand, I was able to correctly guess these words that I'd never seen before:

omphaloskepsis

crepuscular

absquatulate

callipygian

houghmagandy

quire

And then there were these, which were just totally foreign to me:

hippopotomonstrosesquippedaliophobia

nudiustertian

ergophobia

tittynope

Final estimate: ~73000 words