It all depends on how you grow them, or more accurately....how well you fill the grow area. You don't want any open spaces, nor do you want overcrowding. So, basically, you could end up with the same yield, either way, if you manage your canopy correctly. 10 narrow plants will yield the same as 6 that are more spread open(trained, if necessary), but'll probably take slightly less veg time, 'cause they'll be flowered at a smaller size than the 6.