Ok...

I have heard time and again that a temp gun is the greatest with a rack, and that you should use it regularly to check temps.
Well, I have a temp gun and I have a rack... but how in the world do you accurately check the temp in the tub with the tub pulled off the heat source??? Makes NO sense to me whatsoever. When you open the tub, you pull it off the heat. So whatever reading you got wouldn't be accurate.

Sorry to pull away from the OP topic...but that has always puzzled me. I want to get good readings on tub temps, and currently use Acurite thermometers in each tub, but can't figure out the temp gun use thing.

As for the original question, you shouldn't have any trouble using one t-stat for Many shelves. The problem would be if you didn't buy all the flexwatt at once. Different batches of the stuff can have different heating power, especially in the 11" width.

My suggestion with any rack would be to build it as large as you want it to be and wire it up with tubs, even if you won't have that many snakes. That way, you have tubs ready to go for future additions and all your flexwatt is the same.