When logged in, go to your site stats (either get to it via your dashboard and the menu in the left panel or via the grey admin bar at the top of your window and then, from the menus that come out via your name top right, choose the blog and its respective site stats). When on the site stats page, scroll down to the bottom til you see 'Totals, Followers and Shares' and look at the 'Followers' area ('Followers' is what used to be called 'Subscribers', it was changed fairly recently). You'll see the total number of followers in a text link. If you click on that text link you'll get up a page that will show you the people who are subscribed. There are two text links at the top of it: "wordpress.com followers" - these are people who are subscribed via the site, usually logged in users with wordpress blogs who can read their subscriptions on a reading page here but who also have the option to receive posts via email. With these you can click on the upper link which is to their public Gravatar profile or the lower link which is to their blog. The other link is to 'Email followers' and are mostly people who either don't have blogs or who have a non-WordPress.com blog (for instance, they might blog on WordPress.org, which is different from WordPress.com). There are no links from the email followers list, but you can see the email addresses of those subscribers.
As for whether everyone who is subscribed has actually received your posts... you won't find that on the subscribers lists. If you've had people tell you that they are not receiving your posts, it may be that they are being intercepted by their email spam filters (it happens with most blogs from time to time, not just wordpress), or it may be a problem with their internet provider or software. Or it might be something glitchy with WordPress.com. If you can't sort it out with the subscribers themselves, I'd suggest you contact staff at WordPress.com Support:
Just type a few words into the form there, and if suggestions appear that are no use to you, then type a full description of what is wrong and send it off to them. Wait a while for a response as they can get very budy. (I'm not a member of staff, by the way, most of the people who respond to topics on this forum are volunteers.)