What happens with only 1 vent?

Do I need both a vent to suck out and and bring air in? What if I just have one air venting out and a single oscillating fan in room?

The reason why I am asking is because I am installing a vent on my ceiling to get the air out...but if I decide to install another vent, it will also have to be on my ceiling...this ill be sucking air in that I expel out...BTW its all going to an attic which has vents that go outdoor...and I am not worried about smell for now...

What do you guys think I should do?

well the best thing to do would be have intake and exhaust but some people just do one. personally i have no exhaust to my grow room ( i assume it just goes out under the door) i use a air conditioning unit bringing in 73 degree air through my window for co2. i guess it might be counted as exhaust i have a fan running 24/7 in my kitchen window which sucks air out of my apartment and i keep a damp towel at the bottom of my entrance door to keep the smell from seeping out into the hall.

You will need some way for fresh air to enter the room. If it was airtight then the fan could not suck air out efficiently. You need to keep the air in the room fresh, aim to cycle it every 5mins, more often (every 1-2 mins) if your using HID lighting, as you'll have to keep temps down. Fresh air=healthy plants.