It matters on how much smell you need to cover. If you have visitors you'll probably need a scrubber. if you live alone without visitors then the smell probably wont make it out the door. or you might get away with some ona gel or something. 2-3 plants wont put off too much smell but it also depends on the strain and how big they are. 2-3 lil plants in a pcgrow = covered by ona gel easily...2-3 5ft monsters = carbon scrubber.
If smell is an issue there are low/no odor strains out there you just have to look for them.
I'll have 3 plants if I'm lucky (based on my fledglings now). They're bag seed, so I have no idea of the strain. I have no way to vent my closet for a carbon scrubber to blow the air out.
I did however discover that my DeLonghi Air Purifier (
http://www.air-purifier-power.com/delonghi-air-purifier.html) has a carbon filter as well as a HEPA quality regular filter. It also has an ionizer, but I probably won't use the ionizer.
I'm hoping that with a few cans of odor absorbing gel, the DeLonghi in the closet will work, esp. if it's constantly on and set right in the closet.
I have an extra DeLonghi too, which I can run in the house, plus maybe I'll put a few cans of odor absorbing gel around the house.
Plus -- maybe I'll flower and harvest before the plants are monsters (if I'm lucky enough to get'em that far).
My biggest problem is that the closet is right next to the front door of my place, so anyone standing outside the door might get a whiff unless I'm careful. And any visitors (maintenance guys, etc.) will walk right past my babies, so the odor has to be completely covered.
I tell ya, the more I learn, the more I see why most people don't grow their own. But I really don't like drug dealers (and I really like weed), so I guess I'll stick with growing if I can.