Add as required untill you have to wear sunglasses then unplug one.

Seriously though you'll have to add as many as you need. One directly over the work benches is great and you can fill the outter edges as needed. They throw out a good bit of light. One of these has more usefull light than a normal 8' twin bulb fixture. These are not a "dull" light like the 8' . It's really crisp.

We have 10 8' twin tube lights in our transmission building room (20x40 10' ceilings) and we still had to use flash lights at times to see our work well. I added a single T8 fixture over each build bench and now we have plenty of usefull light.