Well on Talking Dead , Greg N. claimed this was the biggest amount of extras the show has ever used. That it was a record setting deal as far as extras go for the show thus far . This is something he hinted about weeks ago during the pre-show before Fear the Walking Dead. I do think he used some CGI as well to show the massive quarry pit of walkers.
As he joked about the end of the show , "Well some dude came by and blew his horn , claiming we done blew the entire budget on all the extras. So that is why we ended it that way."