6-8 bulbs...the exact output will depend on the K rating of bulbs you choose. A bunch of bluer bulbs will be less PAR than if you have a bunch of 6500Ks...but the look will be different as well. So it depends. It also depends on what you plan to keep...but thats been mentioned already.
It also depends on how you arrange the bulbs. You can use two banks of 3' bulbs, 6-8 bulbs each. You can use 8x4' bulbs, and stagger the ends to make 6' (bright in the center, softer at the ends), you can use 5' bulbs as well....but this will mean limitations and such as there are less bulbs available (at least at the same price as 4' bulbs) and reflectors. Heck, you could do 3 banks of 6x24watt bulbs (2') if you want...but that would be too expensive to make it worth it.