I'd suggest RS485 from the master out to the remotes, with the remotes having intelligence to receive a serial message and manage the relays from there. Way less wiring, each remote can be powered locally. Remotes can then report status too if you want to set up sensors, or have light switches that can be turned on manually at the remotes sites, temperature, etc.
Can make a little custom card with equivalent of a Promini, RS485 chip (MAX488 perhaps), wallwart for power, relay driver (shift register like TPIC6B595) and that can also control other things, some sensors, etc.