I'm not sure how you plan to send the data between the various Unos and the Mega, but for what I am going to say it doesn't really matter.
I suggest you arrange for the Mega to send a message to each Uno in turn and then listen for a reply. That way there is an organized system that is easy to control. Each Uno can have some unique "address" so even if they all hear the message only the correct one will reply.
By the way there are cheaper options than a Uno if you need a lot of them.
...R