How can I find out how many people received my newsletter or email?

Go to the Reports menu > section 'Email/Newsletter' and run the Email & Newsletter Stats report (make you also tweak the interval for the report, if the case). This will display various information regarding your newsletters and emails: how many contacts the newsletter (or email) was sent to, your open rate and click rate, how many emails bounced and also how many unsubscribed. You can further click on each percentage number in the columns to view the list of each statistic.