I've been trying to remember the word for "knowing the quantity of objects in a group without counting them" for years. No exaggeration. I've never been able to formulate a question to Google so it actually knows what I'm talking about, and no people I've asked knew that that concept had its own word. And I finally discovered it again this morning.
It's called "subitizing".
And I learned it from this SCP article: https://scp-wiki.wikidot.com/scp-3755