There are several ways to determine the amount of light you need for a given size room. One way is 5000 lumens per square foot of space minimum up to a maximum of 7500 any more than that and you will stand a good chance of "sunburning" your tops.
The other way is 50 watts per square foot minimum up to 75 watts per square foot. Using either formula will give you a very close idea of what your basic needs are to flower good plants.

Vegging plants is a different story all together. All you need to veg 9 plants is one 4 foot by 4 bulb T5. I just did 12 clones for my present grow under one such light that were 32 inches tall (topped) when put into flower after 60 days.