In Iowa the law is about how many can be on at one time, and doesn't matter how many are showing just as long as not actually turned on while on the road.
What you might want to watch for though is if you go into other states, take IL for example they not only can't be turned on, but must not even be able to be turned on from within the cab while driving on a public road. I.E. you have to need to get under the hood and hook them up in order to run them. MO I believe they have to be covered up etc. So it's not just a consideration of what Iowa law states but also what any state you travel in also requires.