Mail puts all incoming email into your Inbox; the statistic after the word Inbox lets you know how many messages you haven’t yet read. New messages are also marked with light-blue dots in the main list.
The Mail icon in the Dock also shows you how many new messages you have waiting; it’s the number in the red circle. In Mail→Preferences→General, you can specify which mailbox it’s tallying up—or have it tally up all of them.
Click the Inbox folder to see a list of received messages. If it’s a long list, press Control-Page Up and Control-Page Down to scroll. (Page Up and Page Down without the Control key scroll the message pane instead.)
If you ⌘-click more than one mailbox folder, you can ...