"Slide deck" is an idiomatic phrase in English, which means a collection of slides. "1,200 startup slide decks" means 1200 collections of slides. If there are

"Slide deck" is an idiomatic phrase in English, which means a collection of slides. "1,200 startup slide decks" means 1200 collections of slides. If there are only 67 decks, you would say "67 startup slide decks."