Will I get an email for every single comment?
Not unless you want to! Site authors have a profile option to receive all comments, but that's not the default for other users.
To access this setting, go to Settings > Postmatic, then visit the Configure Posts tab.
Postmatic aims to send you content you want. When you reply to a post notification, you become a part of the discussion on that post and receive other comments on it. You can reply to any of these with 'unsubscribe' to leave the discussion.
You can even send a blank reply to a post notification to receive new comments without leaving one.
If you don't reply at all, though, your inbox will remain unclogged by discussions that don't appeal to you.
Postmatic also has built in comment throttling that you might want to know about. You can find out about that here.