ERA-DCKIT-2 
The only real limitation with the ERA series of products is the number of zones. As mentioned in a previous FAQ article, you can pair as many transmitters to a specific zone as you want.
There's no limit to the number of receivers you can have in your system. Whether you have 1, 2, or 4 different transmitters, you can have as many receivers as you want and can spread them throughout your warehouse, store, or office space.
This type of configuration allows you to distribute the sound throughout your facility. You may, for example, play a receiver in the warehouse and turn the volume up to its maximum setting. However, because you might keep your door shut in your office, you may want to put your ERA-DCRX receiver in the office and turn the volume down--or even mute it.