IMO anything more then 6 tubes wouldnt get you much saving in terms of energy and $.
I am using 6 bulb T5 Tek and just to replace 6 bulbs after 20% discount i paid $164.60.
regular price on a 48" tube is $34+tax
if you get
10 bulbs you are pushing up to 300 or more depending on your tube preference.
For a 48" tank you can get very good SPS growth and color with two 250W MH for a lot less .