How many bulbs?
 

I am converting an aquarium to better use, so I can grow some orchids in my office.

Dimensions: 48" x 21" x 12" (L x H x D)

I plan to have Paphs & Phals on/near the bottom,
and some small Vandaceous plants mounted higher up.

I am going to buy a fixture with 48" T5 tubes. Do I need 4 tubes, or will 2 be enough?

How many kelvins are your t5 bulbs? I'm growing all of my orchids (vandaceous, dendrobium, cattleya, and maxillaria) under two lights that each are about 4,500k. Everything is in vegetative growth right now :) which is fine by me as most of my plants are too young to flower. When they're old enough to flower, I may switch out one of the bulbs for one with warmer tones (lower kelvins) to promote flowering. So for whatever worth this long anecdote is for, I hope it helps! Also, a factor would be how far away you keep your lights from the plants. Mine sit above the rim of my ten gallon aquarium turned terrarium so the plants are maybe 14 inches below the two lights.