You can have a total of 8 TPL-420E adapters on a single network.

Troubleshooting

Please reference the items below if you continue to experience difficulty with installing and using your Powerline adapters.

• Power-cycle the Powerline adapters by disconnecting them from their power outlets for 20 seconds and then reconnect them. 
• If the Powerline (PL) LED is red, try plugging the Powerline adapter into another available power outlet.  The quality of the connection may be better in a different power outlet.
• If you are having issues connecting the Powerline adapters after changing the encryption key reset them by pressing the reset button for 1 second.   You can also use the Powerline utility to set the Private Network Names manually on each adapter.
• Press and hold the reset button for 1 second on each Powerline adapter. All LEDs will turn off then turn back on. This will reset the Powerline adapters to their default settings. 
• The Powerline adapters can provide better connectivity and performance when plugged directly into a power outlet.  Connecting the Powerline adapters to a power strip or surge protector may degrade or block Powerline signals.
• Avoid using the Powerline adapters on GFI protected outlets and electrical systems with AFCI circuit breakers.  They may degrade or block Powerline signals.
• The Powerline adapters should not be used in areas with excessive heat.