Short answer
Sales > Activity Feed lists your sales notifications from the last year. HubSpot's KB says it shows email notifications only for tracked emails, and real-time notifications fire when a lead revisits your site, a tracked email is opened or clicked, a document is viewed, or a meeting is booked. The number of notifications per month depends on your subscription.
1. The feed
My demo portal shows the setup prompt and HubSpot's sample activity.
2. Read signals carefully
See open notifications for why open counts can mislead.