badtzphoto asks When do you use "got it" vs "got them"? (ex: 3 unrelated points, do I say "I got it", "I got them", or "I understand"?)
We've actually got two different usages here, a literal one and a colloquial (slang) one. With a little help from our friends in The Sentinel
Let's take the literal one first.
'It' is singular, meaning one item. 'Them' is plural, meaning more than one. So 'Got it' or 'I got it' means you literally did get (receive) one item."Did you get the book I sent you, Sweetie?" Naomi asked.
"Yes, I got it, thanks," Blair answered.
After they spoke for a minute longer, Naomi rang off, Blair put the phone down, and returned his attention to his new book.
'I got them' means you got more than one item."Did you get the books you ordered?" Jim asked.
"Yes, I got them," Blair replied.
If you're speaking colloquially, 'Got it', 'Get it', 'I got it' or 'I get it' means 'I understand' or, if it's clearly a question, 'Do you understand?' This usage, because it refers to a piece of knowledge or information, is always singular, so 'I got them' wouldn't make sense."I told you! I've tried!" Daryl exclaimed. "I've tried, but I just can't make sense of it! Got it?"
Simon sighed. No use asking Blair if he could help Daryl to 'get' how the mathematical formulae worked; it wasn't his subject.
In speech, the colloquial form is perfectly acceptable; in narrative, however, you would normally say 'understand'. Simon could indeed understand that Daryl found math difficult. He, too, had found the subject very difficult.
You'd only use 'I got them' if you'd physically received two or more items. Pretty well any other time '(I) got it' would be correct, though in narrative it would be better to say 'understand'.
The best reference I could find for 'got it' aka 'I understand' was on a blog,
here