Lack of details prevents us from answering accurately.

What are these circuit going to be used for?
If they are inside a home it is most likely that they will have to be AFCI protected, so you would need a 12/2 for EACH circuit.
If not then a 12/3 could be used to share a neutral between TWO circuits.

It kind of worries me that you are asking about sharing a neutral between four circuits. This shows a real lack of understanding of this work.

With 12/2 cable you need four neutrals.

If the poster is installing 12/2 cable for each circuit what is he/she planning on doing with the grounded conductor(neutral)(white) from each cable?

I guess I posted wrong (I was posting for my husband). To clarify, he was told he could run four 12 gauge single conductor wires and one 12 gauge neutral single conductor wire. This is going from a basement entrance panel, outside the house thru conduit to power an old existing entrance box that is upstairs. The upstairs entrance panel will be turned into a junction box. It has four 12-2 wires without a ground. The question is, is it possible to run one neutral to take care of those 4 wires. (Or one bigger neutral wire). Also, what size of pvc conduit should be used for the amount of wires used.

"is it possible to run one neutral to take care of those 4 wires."

In short, NO. You can not use one neutral with 4 branch circuit conductors (unless two of them feed a 240v load). However, you hve left out alot of information that would be necessary to give a more detailed response.

kristine there is no such object as a 12-2 wire.

A wire is a wire or conductor as usually designated by its gauge, the metal its made of and whether its stranded or solid. A few other designators go into the type of insulation surrounding the wire.
For example: have four 12AWG, solid copper wires running into the junction box. AWG is American Wire Gauge.

As you are describing 12-2 that usually means a cable with two 12 AWG wires withing the cable sheathing

you say in your last post, "The upstairs entrance panel will be turned into a junction box. It has four 12-2 wires without a ground."

Do you mean four 12/2 cables or 4 12 AWG wires

With four hot wires it is possible to use only two neutrals if it is wired properly at the panel.

Kristine, why is your husband not posting here. That would clear things up a bit.

Second, whoever told him he could run one neutral for four circuit has NO place giving electrical advice as it is obvious they have no idea what they are doing. I fear your husband knows even less.

PLEASE make sure he is certain of what he is doing BEFORE he does it, and double check everything you are told.

While there's still a few 240V loads (e.g. window A/C) that can be wired w/ AWG12, I think I smell an overloaded grounded (neutral) conductor... OP should install extra smoke alarms while they're at it... Mr. Bill! =:O

Agree there's MANY critical details missing, incl. type of existing conduit (metal or ??), associated GND bonding system, opposite leg CBs, etc.

I know copper is spendy, but so is a house fire, and there's no better way to bypass the safety of a breaker than by OVERloading a neutral.

Thanks everyone for the replies. My husband wasn't able to post as he has been out of town this week. He had been talking with someone else about this project, and they had said he could get by with one neutral. He didn't know how that would be possible which is why he wanted me to post here. Sorry that my lack of electrical terminology confused the situation. We appreciate the responses.

Just re-read your original post...
I am needing to power 4 separate circuits...

Which strongly implies four 120V branch circuits, this requires a MINIMUM of 2 neutrals, and that's only possible if the 2 "hots" sharing each neutral are powered by opposite "legs" of the main panel.

A good illustrated text on residential wiring should clear up any confusion. Do NOT proceed until you clearly understand the responses in this thread... or call a pro... with references... and pull a permit... permits include INSPECTIONS... always good options.

It sounds like your hubby may be getting bad advice, perhaps deliberately so, by someone who wants to bump him off, and move in on the grieving widow... LOL... I kid, I'm a kidder. ;')

while your husband may be a great guy, he is clearly not a licensed electrician and therefore should not even be attempting this work. I have a bumper sticker on my truck and it reads as follows: 'wiring is not a hobby...hire a licensed electrician.' if he chooses to do the work himself, tell him to make sure that he has the number to the local fire department close by.

Watch the box fill, too!

"while your husband may be a great guy, he is clearly not a licensed electrician and therefore should not even be attempting this work. I have a bumper sticker on my truck and it reads as follows: 'wiring is not a hobby...hire a licensed electrician.' if he chooses to do the work himself, tell him to make sure that he has the number to the local fire department close by."

Pretty useless advice on a DIY site.

oh geez it isn't brain surgery. even the theory behind it is well within the grasp of anyone willing to learn. and like brickeyee said, this is a DIY site, right?

if he chooses to do the work himself, tell him to make sure that he has the number to the local fire department close by.

It does sound like this particular DIYer has some challenges. However, as a friend of mine used to say, "Stupidity is forever, but ignorance can be cured." A copy of Wiring Simplified will get this guy on the right side of the road in a few evenings' worth of not-so-light (sorry) reading.

Since this is a DIY site he will have to go to the firefighters forum and learn how to put the fire out himself. No calling professionals here. Just incase the DIY turns into a DYI (do yourself in). Whatever you do don't admit defeat and call a professional, at least untill you smell smoke, or your meds kick in and you have a lucid moment. Here is a tip. Lots of insurance and don't let anyone reside there that you care about. Winning!

"Whatever you do don't admit defeat and call a professional, at least untill you smell smoke, or your meds kick in and you have a lucid moment."

The OP (or his stand-in) is asking questions.

They have been told they need to understand what they are doing, but recommending that only "he is clearly not a licensed electrician and therefore should not even be attempting this work" is a little over the top.

Wow, this is a real friendly DIY forum. Don't know if anyone noticed, but this is an old thread. Not real sure why it is continued to be discussed. I posted this question basically to settle a difference of opinion my DH and another guy had over this project. They had been discussing it at work and he was told "he was told he could run four 12 gauge single conductor wires and one 12 gauge neutral single conductor wire". He obviously questioned their judgement, which is why I posted.

Thank you to those who offered sound advice and not just ridicule. For those of you concerned with our stupidity and well being, we did hire a professional to do this job. I will definitely think twice before ever asking for advice on this forum again.