How do I know how many people received and opened my push notification?

Open Plugins → Push Notifications and scroll to the Delivery log. The top right shows how many subscribers you have, split into web and app. Each row shows:

• When it was sent and its Title
• Channel (web or app)
• Sent: devices it reached
• Failed: devices it couldn't reach
• Opened: how many people clicked it, with the open rate in brackets

Compare Opened across your sends to see which titles and offers get the most clicks.