all the relays to switch simultaneously
If you are controlling each relay with a different Arduino pin, you are actually turning them on sequentially, just very quickly sequentially. So fast sequentially it might as well be simultaneously, but still.
I'll show myself out.
I really doubt you need 2 amps but here is a 5v 2A supply:
Nit pickers: Something about port manipulation, assuming no set up where everything runs through one transistor or something, other nitpicky things- you get the point.