Maximum Number of Zones

[HereComesTheRain]

Great to see you are almost there!

The power your Pi can deliver on 5v depends on the power usage of the Pi itself and the psu you have connected to it. Also USB devices, if any, will consume from the 5V power.  If the Pi is not doing some heavy stuff there is likely to be enough power to drive all inputs at the same time.
Assuming you have 4 relay boards of 8 channels, that makes 32 * 20mA = 640mA. I think the 20mA is in reality a bit lower.

So I'm fairly sure that if your Pi is running SIP only and you have no USB devices connected you can drive all channels at the same time. There are however fuses on the Pi which eventually can blow when overloaded.  If you are really in to pushing the limits I can do some more precise calculations/estimations.

--Gerard

Using a PWR+ charger, mfg states 5v 2A 2.5A 3A 3.5A but no way to verify. In case I run into problems can you recommend a robust power supply?

USB wifi adaptor uses 340 mA when TX and 160 mA when RX.

Breadboard w/ 4 shift registers attached.

Running the Pi OS Desktop Headless, SIP, APCUPSD (maybe that will tell me what the Pi is drawing), no keyboard or mouse attached.

So I guess I am ready to move on to connecting the irrigation valves to the relays and the third power supply.

Thank you again.