The idea of using wattage as a metric is N watts per square foot of grow space. The underlying idea/goal is that the entire grow space will be filled by the canopy of the plant(s) in the grow space.
Input wattage is only a rough metric. What drives crop yield is the number of photons hitting the canopy. A grow with a modest input wattage, say 30 watts/square foot, can have a great yield if the tent is full whereas a high powered light that's hitting a plant that fill just a limited amount of the grow space is "wasting photons".
The number of plants does impact yield per grow as well as yield per year. If you grow many small plants and cover the grow space, you might get four grows per year whereas, with larger plants that don't quite cover the grow space, you'll get a lower yield per grow and, since they take longer to grow, you might get only three crops. That's not a big deal if you're a personal grower but something to think about if you're growing for others or if you're in the biz.
In terms of input wattage, 30 wats/sq ft is "modest", my preference is for 40 but I will be upping that to 55 watts/sq foot starting with my next grow . The reason for that is that light is how a plant makes food ("nutrients" are not food for a plant - they're akin to vitamins and minerals) and, up to "the light saturation point", the more photons hitting the plant, the larger the crop and the higher quality of the crop, in terms of the amount of secondary metabolites as well as in terms of the ration of flower to plant mass.
