This question already has an answer here:
I've often been curious how really large crowds in movies / TV programs are filmed. e.g. in a stadium at a big football match or something, I'm guessing they don't simply hire tens of thousands of extras! (Or maybe they do? But I'm pretty sure I've seen this even in relatively low-budget works).
Since this predates CGI, a non-CGI answer would be good!
If it's a kind of copy-and-paste going on, it would be good to have an example so to try and spot the repetition...