How do I know how many guests actually opened my album?

Several ways, depending on how much detail you want. Each album card in your library shows its guest count. The dashboard has a "Guest Engagement" stat covering the last 24 hours. And "Show Guests List" on any album opens the full register — every guest who signed up for that album, with their details. So you can see the big picture on the dashboard and drill into individual albums when you want names and contact info.