It does not matter whether the relay is powered from the Pi or independently. You will however need sufficient power for both devices, a 3A PSU should be sufficient.
Yes the GPIO pins are limited to provide 16mA individually and no more than 50mA or so in aggregate. You cannot address this limitation without some transistor circuitry to provide current gain....
You may want to check out this Instructable: https://www.instructables.com/Multiple-Raspberry-PI-3D-Scanner/
He initially connected 40 Pis together using Ethernet to perform body scanning.
In a update (Step 8) he increased the count up to 90 Pis tied together:
check in file explorer with which letter (that C:// or D://, don't know what it's called) it starts and make sure windows recognizes it. and you need not format the card as it has binary junk because the OS is only installed on it. OS installation removes the formatting.