Hi Sean,
I have had my setup for about 5 years now. I have always used 1 Pi to each Sandevices controller.
If I recall correctly, I have seen some hobbyists using a single Pi 4 running multiple controllers using a network hub. I can see the cost savings in a single Pi 4 configuration, but is it better to have 1 Pi responsible for say 4 E6804s and 2 E682's, (with a backup Pi as a backup) or should the workload be divided up into a Pi per controller? Just curious, as I'm going over all hardware to have the best configuration. An overview of my show is: 1 3600 pixel megatree, 3 singing faces, 4 minitrees, 4 windows, a roofline a pathway 4 arches and a few stars. I hope this paints a decent picture for you. Thanks in advance!