Ok Here is what we know from what is on screen:
LV-426
did have an atmosphere to begin with, but was comprised of unbreathable gases- methane being one of them.
Van Leuwen said that there had been colonists on LV-426 for at least 20 years who go in and set up "these atmospheric processors to make the air breathable"- and that it "takes decades".
Also from the inquiry scene- LV-426 is
a rock with
no indigenous life.
So the impression that I got is that the colony on LV-426 is a mining colony and that Weyland Yutani is primarily a mining industry (despite their slogan). This would tie in with the Nostromo being a tow ship with 20 000 000 tonnes of mineral ore. Not an agricutural colony. If there are colonies on distant planets that are there to grow things they are not run by Weyland Yutani. WY is the proverbial villian in this series- likened to any multi-billion dollar industry that couldn't care less about it's workers or the effect it might have on an ecosystem.
As already stated- Ripley asks "Is
that the atmospheric processor?" and Burke answers "Yep
that's it". No plural- no big explanation that there is more than one (however he does start spouting Company propoganda about what a marvelous piece of machinery it is).
Sooooo...
I think Burke would have mentioned there was more than one processor during his rambling as he was so proud of it and the Company (or he is just used to brown nosing all the time and doesn't know when to turn it off).
When Van Luewen said "these atmospheric processors" this could go either way. He either meant multiple processors on one planet (LV-426) or a
single processor on
multiple colonies stationed on different planets throughout the galaxy. I believe the latter is the case here.
The processor was designed to make the existing air breathable- not create an atmosphere. A reverse of pollution and global warming etc.
It was also said to "take decades" and that the colonists on LV-426 had been there for 20 years. There is nothing to indicate that the atmosphere had been breathable for any length of time- it
could be that the atmosphere had just become breathable prior to the incident with the Jorden family (perhaps in the last year or so) and that the colonists had to wear enviroment suits before this.
I think the writers really got it wrong by making LV-426 only 1200km in diameter. This is approximately 1/3 the size of our Moon. I don't truly understand physics enough to know if a smaller planet could have an equal (or at least close) gravitational pull if it were made of a material denser than that of what Earth is comprised of.
Bottom line is that there is nothing to suggest there were multiple processors- it is all
speculation that it would take "too long" for one processor to alter the atmosphere of a planet (talk about an absence of evidence

). However there is evidence that the concept of multiple processors was changed to one processor in the script drafts.
It's a sci-fi movie. There is something called the "suspension of disbelief" that has to come into play here methinks.
