An Alien Nest is comprised of a group of alien eggs, it's unclear how many eggs and one Xenomorph can lay. But it's clear Sevastopol's first Alien, laid quite a few of them. With only a handful making it to adulthood before Amanda Ripley arrives.
They're made of large groups of Alien Eggs, so they're weak to the Flamethrower and Stun Baton. But eggs seem to hatch only when Ripley, or other Humans, get near. So it's not ideal to use a stun baton unless you drop, or spawn, next to an unhatched egg.