Two years ago, Sequent Microsystems introduced a stackable 8-Relay board enabling up to 64 relays to be connected to a single Raspberry Pi board. The 8-relay board only supported 24V/2.5A, and the company is now back on Kickstarter with a 4-relay board with 250V/10A line-switching relays that can offer up to 32 relays by stacking 8 boards connected to one Raspberry Pi board via the 40-pin I/O header.
Source: Connect up to 32 Relays to Raspberry Pi with a Stackable 4-Relay Board (Crowdfunding) (CNX Software – Embedded Systems News)
Also: Stackable Raspberry Pi add-on has four switched relays (LinuxGizmos)
