What is the correct response when someone says to you
Saludos a todos
You can reply with the usual forms hola
buenos días
, etc...
Even if the speaker seems to be trying to reach a broad audience, you don't need to go with anything special, like "saludos a ti también" or another "saludos a todos".
Imagine yourself in a room with other people, when some else joins the group and says saludos a todos
(like a teacher who enters the classroom or someone who joins a meeting late). You just need to say "hi" to him or her. You may have already said "hi" to the other people in the room, or maybe you don't even need to do it.
In Chile we always reply
En su nombre... (formal)
or
En tu nombre... (informal)
which roughly means "I will greet all in your name".
Usually you use "Saludos a todos" if you say "good bye" to somebody and want to send greetings to his acquaintances or relatives.
If somebody says to you "Saludos a todos", you can simply reply with "gracias". Another more formal reply would be "gracias, serán dados", meaning that you comprise to give the greetings.