Here is something interesting that you might like if you are into DIY stuff, IOT or crypto mining - a custom 28 relay control board designed for use with Raspberry Pi 3. The Pi has 28 pins that can be used as GPIO ones and thus can control up to 28 relays...
The board was designed for use with mining rigs and to be able to control up to 28 separate ones, though later on I have discovered that 3 out of the 28 GPIO pins on the Pi 3 have a bit of erratic behavior when turning on and off the power to the Raspberry, so essentially only 25 can be used problem free. The other 3 might be fixable with some more extra work figuring out why the Pi 3 turns them on and then off when power is applied.
A simple and effective solution to a problem as there are no affordable and commercially available relay boards that come with more than 16 relays (probably the demand for such is not big anyway). If you want to use the full potential of a RPi however you need a bit more relays and 25-28 seems to be just about the right number. Anyway 4 controllers can handle exactly 28 relays, so why not put them all... :)
If you have a question or want to add something, then please leave a comment below.