Can I safely connect 20 Raspberry Pi Zeros to this 5V 10A power adapter
Just be aware of what happens during a failure.

If you short circuit something inside the rpi, or powered through the rpi, then >10amps will flow through the rpi, cables (e.g. usb cables) etc

Do you want this to happen? Can it turn the fault from "crashes program" to "destroys rpi" or "burns down house"?

I would power each individual rpi through a polyfuse.

Also be aware that a 10A power supply likely means "not more than 5A running 24/7"



5V/10 amps used to be a fairly safe level when shorted, as the voltage drop across the wires would be very small, and the psu would shut down. However now cables like usb cables have such thin copper that there can be significant resistance, a large voltage drop so the psu does not shut down, and enough heat generated in the cable to melt the plastic.

When I tested some cheap USB cables a while ago, they caught fire at ~5amps. This was not surprising when I cut them as the conductor was fuse wire thickness. The plastic was not self extinguishing