Most of the research I did shows that people use either one or two regular "shoplight" fixtures per shelf. Each fixture has 2xT8 or 2xT12 standard fluorescent bulbs.
In the past, I used only one of these shoplights per shelf and I felt that the seedlings would have done better with more light- but I got by. It also depends on how wide your shelfs are.
This year, I don't have room for a 4ft long shelf assembly, so I am using a fish tank stand that has two shelfs and is 30in long. I needed 2ft light fixtures and had a choice between a light fixture with 1xT5HO bulb or a fixture with 2xT5HO bulbs. I decided to splurge and go for the 2xT5HO bulb fixture. T5HO fixtures are more expensive, but they emit more light and run more efficiently than T8s/T12s. I think the 2xT5HO bulb fixture is about equivalent to two side by side 2xT8 (or 2xT12) shop lights (4 bulbs total).
It may be overkill, but in my zone, the seedlings may end up stuck inside longer than I would like, so I figured more light is probably better- I can always regulate the amount of light by the number of hours I leave them on.
I plan to purchase 2 fixtures, one for each shelf. I've only gotten the first one so far.
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