There are a couple of ways to do this. This first way is to run 6 pieces of 4ft heat tape and buy connectors for each strip. Then plug those connectors into a power strip and then plug the power strip into the thermostat. Some people like to do this because it allows easy access to replace the heat tape if you leave the back open or easy to open (able to unscrew it or to use a hinge/lock system to keep it closed until you need to open it). With this method, you have to keep track of temps on each shelf in case your heat tape goes out. If the heat tape goes out where your thermostat is, then your thermostat will kick the heat tape into full power and cause the other rows to get too hot. Because of this, I don't like this method.
You can also cut little 4 inch grooves so that you can run the heat tape from the top shelf to the bottom shelf in one continuous piece of heat tape. You would probably need probably need 30 ft of heat tape. This allows the heat tape to pass along the edge of the shelf between the shelf and the side of the rack. You would only need one set of connectors and your probe would go in the middle of your rack. I personally like this method. The downfall is that if the heat tape goes out, you would need to replace all of it instead of just a small section like in the method above. In addition, sometimes there is a variance in how the heat tape heats up, so you could have a slightly different hot spot on each level. Usually the bottom is the coolest with the top the warmest.
As for the THG heat tape, ALL my racks have it and I love it. I have one homemade rack with the heat tape and connectors in two pieces (but I have each on it's own thermostat since I was using the rack for two different species) and I have two RBI racks that come with the heat tape already in the rack. Heating is pretty consistent throughout. The connections come with a cord that you would plug into the thermostat and the thermostat would plug into the wall. Make sure you connect the connectors to the heat tape and get a good current running to use your heat tape to it's max potential.
Also, just to put myin regarding thermostats, I have a Herpstat 2 and I LOVE it. But just for one rack, the Herpstat Basic or Herpstat 1 should be fine.