That depends on your veg/flower schedule. Are you planning on growing trees? 18" plants then flower. I mean really it depends on how your doing it. my veg space is 3'x1.5' and 3' tall.
But i only ever have 2 plants in full growing veg, 1 bonsai mother and 2 clones waiting for the vegging plants to get moved to flower room. Once they get moved, I xplant the 2 clones, take 2 new clones and in 2 weeks i do it again.
but honestly, veg plants dont really smell. at least nothing a spray deodorizer cant clear out in the case of company. 3 vegging plants under some cfl with 24 hour lighting. All you need is a shelf to put them on and hang some lights.