There are 40 hordes to take down in Days Gone. It is mandatory that you destroy three of these hordes as part of the main story. Smaller hordes will only have about 50 Swarmers whereas the most dangerous hordes can be comprised of up to 500 Swarmers.
Although hordes have a specific location where they will hibernate during the day, at night they are out and about. Thankfully, once you've unlocked the Horde Killer Storyline, if you get close to the location of the horde, it will show you where
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