​→ To find out how many subscribers you have, go to settings > subscription: 

→ To get the list of subscribers, create a list of contacts in Marketing > Contact lists and choose the condition Contact properties > Subscriptions > Is equal to > Name of the newsletter. You can then export or consult this list. 

→ To monitor your newsletter subscriptions, go to your dashboard (blue button): 

You can then access your email-sending statistics and see changes in your subscriptions

Move the cursors in the graphics columns to see the changes.