How many Optex TC-10U Door Switches can I use together?

The Optex RC-20U wireless chime receiver can be connected to up to 12 Optex Wireless Transmitters. This covers the TD-20U, TC-10U, TS-10U and TR-20U. View all of the Optex Wireless 2000 products here – http://www.absoluteautomation.com/brands/Optex.html