Where do subscriptions to my newsletters go?
If you haven't integrated your theme with Mailchimp (or another subscriber alternative) then subscribers to your newsletter are all in one place in your account.
Click 'marketing' from the left hand menu and then 'newsletter subscribers'.
Here you'll be presented with a list of all people that have:
i) Subscribed to your newsletter using the form on your website
ii) Customers that have registered for an account and said they are happy to receive marketing communications from you
iii) Customers that have bought form you (regardless of whether they created an account) and said they are happy to receive marketing communications from you.
This page has a number of different features that are described on our article about managing subscribers.