How do I see how many people opened or clicked my email campaign?

Go to Marketing → Email Campaigns and click View on the campaign. The top of the page shows:

• Recipients: how many people it was addressed to.
• Sent and Failed.
• Opened: how many people opened it, with the percentage of sent emails in brackets.
• Clicked: how many people clicked a link, with the percentage of sent emails in brackets.
• Sent at: when it went out.

Each person is counted once, however many times they open or click. Opens are recorded when the email's images load, so readers whose email app blocks images may not be counted as opened.