The film script for Aliens indicates that there were 157 colonists, plus Rebecca "Newt" Jorden, making up 158 in total.
RIPLEY: But they will know about it, Burke. From me. Just like they'll know how you were responsible for the deaths of one hundred and fifty-seven colonists here --
Although we do see some evidence of a fight between the colonists and the xenomorphs, without advanced weaponry or military training, there's really no good reason to assume that any of the colonists fought 'to the death' during the Xenomorph attack. This would make the maximum number of xenomorphs that the Colonial Marines were potentially facing the full 157.
That being said, in the comic book extension to the films; Aliens: Newt's Tale, we see the takeover of Hadley's Hope. During the fight between colonists and the aliens, there are at least three obvious fatalities, including Newt's mother, dragging the number of xenomorph hosts down to the low 150s.
But there's also mention that the colony had livestock, which means that the total number of incubators is potentially much higher.