Corrected entry: If an alien creature comes from an implanted human, there would only be as many creatures as humans, but there are far too many aliens. Remember the tunnels with the automatic machine guns? Those guns, supposedly, killed hundreds of aliens.
Correction: There's a lot of overkill from the sentry guns as they simply fire whenever they detect movement and are unable to account for size, etc. Note that the guns fire multiple rounds just to take out the single empty canister thrown in front of them for a test. So the sentry guns having fired hundreds of rounds doesn't mean they've taken out hundreds of aliens.
Correction: In addition, it's emphasized multiple times that the xenos are smarter than expected (example: the "They cut the power!" "What do you mean THEY cut the power? They're animals!" scene). It's highly possible the xenos only lost a handful of their number to the sentry turrets before working out a means to safely trigger them until they ran dry (TV Tropes proposed the idea of them hiding behind cover and waving their tails to set off the turrets' motion detectors).
Correction: The aliens were bred from the implanted inhabitants of the colony. There were plenty of them to supply the amount of aliens seen.