Maybe we can at least consider a bare minimum?
Hive battle - https://www.youtube.com/watch?v=IySXgTv3eU4
From what I can tell, I got this count:
Live aliens: 2
- 1 seems to takes Dietrich away
- 1 seems to take Wierzbowski
Dead aliens 5:
- One killed retreating into the vehicle that sprayed acid downing one of the marines
- Another tries to get in the vehicle but is shotgunned in the head
- Another dies under the tires
- Another kills the dropship pilot as it moves to rescue them
A 6th might be seen killed when escaping the initial hive battle as well, but it's hard to tell
Turret scene - https://www.youtube.com/watch?v=IS2PtmM9mwU
Dead aliens: 3
They actually rarely show the aliens here, though it's said there are "lots" and that they can't tell how many there are.
Standoff scene: https://www.youtube.com/watch?v=wVmu6uHEDHI
Live aliens: 2
- Hudson seems to be dragged away by one
- Burke is eaten by one
Dead(?) aliens: 9?
- We don't see any definitely explode but a number seem to be at least knocked out which I counted to the best of my ability
It should be noted that the motion tracker seems to conservatively show what seems to be ~25-30+ separate blips
Gorman death scene - https://www.youtube.com/watch?v=aHjRQJZsUGg
Dead aliens: 5
Ripley confronting the queen - https://www.youtube.com/watch?v=RS-x1AzjqLQ
Live aliens: 3 (queen plus 2 guards)
I think even if you consider the "live aliens" the same 2 each time, and you discount the queen and chestburster, there is at bare minimum 25 aliens that had been prowling, and I think a safe conservative number would be closer to 40-45, and we can likely even tack on another 5-13+ if we assume "lots" from the turret scene is a subitizing effort on Hick's part and could have accurately said "a few", "several" or even "a dozen".
Clearly, given the colonist count, we're lead to believe the number is almost certainly higher, but it should also be noted that the xenomorphs are rather tough creatures, so my gut tells me there were probably about 100 aliens. It should also be noted that, for actual production purposes, I think there were no more than 6 actors as aliens at any given scene, so even if we take "reality" into effect, we know there's at least that many.