Newsletter is an automated email sent to your followers. People following sellers can opt in/out of newletters. You'll have a summary in your newsletter section. Personally for exemple i'm sending one to new subscribers, then one every 2 weeks for new products. I can see it sent 181 emails mar 17 - jun 15. People opened 31.49% of them and clicked 2.76%. Can't see more. I'd recommend not sending too many, i noticed some unfollow since i started sending them. I think most people hate that.
Edit: actually when you create your newletters, you can select the recipients. You can see how many approx. buyer reach there.