Need recommendations for lights

[Jorch]

I have been thinking about buying T5's to make my orchids happier. The question is, I don't know how many to buy!

My plants are on a shelf right now, 36" in length, 18" depth, about 18" tall per shelf. The shelf is in front of the south facing patio door, so it does get some light, although not a lot especially over our dark winter. I grow mainly papahs and phrags, with a few oncids and vandas (and misc).

so my questions are:
1) how many T5 tubes do I need per shelf?
2) Should I just buy the 2ft tubes? or spend more and buy a 3ft tube? (I've only seen 3 ft tube offered at one store)

3) I am also thinking of getting a 20 - 30 gallon tank for my miltoniopsis. how many tubes I need for those?

Thanks in advance!! any suggestions are much appreciated!