One strip or two strips per level the entire length of the shelf. One strip in the back if the room you keep the rack in is warm enough to keep the cool side of the tub in the low to mid 80's all the time. Add a second strip to the front to maintain the cool side if room temperatures are too low. This would require an extra thermastat though, or one that can control two or more heating devices.

You can wire them all together in parallel to have one plug or wire each one separately and plug them into a surge protector then plug that into the thermostat. Ralph Davis has some nice videos on YouTube about heating a rack. There are three that run about 10 minutes each as he explains the whole thing. Definately worth watching.