Question: So, for portraits or videos etc - everyone always says to get some light, ie softboxes. Okay, so you search softboxes online at your favourite webstore and find that no two softboxes are the same:

We have softboxes in different shapes and sizes, and in with different number of 'heads'. A head being a fixture for a single bulb. Some softboxes fit just one bulb, others fit as many as five bulbs.

So this means that some softboxes will be incredibly brighter compared to others. For example, they ship with 135W CFLs included, advertised as 675W each - mulltiply that by 5 bulbs in a single box and this seems to be a bit excessive?

**Is there a rule to dictate how many heads are generally needed in a softbox?**

I know I could always not add all the bulbs, or move the softbox further away - but space can be limited, and the head count can affect price.

Answer: 
First of all, buy a head with adjustable power! :-) Moving heads to control emitted light is usually a bad habit - the closer the soft-box, usually the better the result is! (You want to make the light source to be NOT point-like, so the closer it is, the softer the shadows are, and that's the point.)

Second of all: are you asking how much light you are planning to emit from a soft-box - from us? :-) Your use-cases may greatly vary... You can do portraits with relatively low light, or you can do high-speed-sync action shooting of a group on the field! With a group of heads!

And lastly: it is all different if you are using flashes or continuous light for video... The flash emits a relatively high energy density short pulse, but then the head, the soft-box can cool down during recharge, idle time. If you have continuous light, you need much better ventilation, etc.

The easiest to solve your problem is: find a studio nearby, go there, make some shots, check the properties of the heads and soft-boxes there, and then you will have enough data to purchase equipment. (Make sure though that you go to a place with some good quality equipment.)

My advice: in general, an octa soft-box is always a good idea to start with (creates a light similar to what an umbrella creates, but smoother. Have you tried a simple umbrella and a flash?). I would buy a big octa first, then a huge rectangle soft-box next. I always use these extensively, however, I cannot justify buying them, as I can always go to a studio if they are needed.

Also make sure that you buy equipment for outdoor use if you want to use them outdoors...
A softbox is a light modifier…. It affects the shape and quality of light. An Octabox would create a cone of light; a rectangle softbox creates a pyramid. It would be easy to evenly light some ones face using a small softbox the size of their face. It would be easy to evenly light an entire person from head to toe using a large softbox the size of the person. Since the large softbox is lighting a larger area it needs more power for the same exposure. There is a simple way to tell how much power you need: If you wish to overpower the ambient lighting; your light must be brighter then the ambient light. If shooting in the dark, no ambient light, any light you use will have enough power. If shooting in full sun your light must have more power than the sun!