Currently growing 2 plants in a 2x2 tent without ventilation. Since it's a small tent/grow in a temp controlled basement I've been getting by with a small vortex fan blowing fresh air in thru the bottom vent flap.
Temps and humidity are easily monitored & controlled. One plant flowering, the other just starting to flower.
The only issue is smell....the wife is complaining plus I know I can't keep farming without controlling odor, as luck has it I'm ordering a fan/carbon filter from Amazon tomorrow.
If odor weren't an issue I'd continue using just the fan however when you live in a townhouse you really don't want to advertise, even here where it's legal