When there’s two people in that list.
For conversations which include interested parts as recipients, it’s good to reply to all to ensure they get all the updates. But this goes for smaller groups. If you go beyond 7-10 recipients, it might be time to ask if everyone really wants this or if some other communication format would be preferrable (chat group or meeting).
When you want receipts to prove you did your due diligence without relying on IT to dig it out of the other party’s email history should a conflict arise.
When others might be interested in what you have to say. If it’s a discussion, them other participants almost always want to receive what you send. However, if you asking a question about an assignment, don’t, no one else needs that.
