By default, 50 emails per page display in your inbox in Yahoo! Mail. To change the number of emails that display, please follow these steps:

1. Click the Options menu in the upper-left corner, and select Mail Options from the pull-down list.
2. Click on the pull-down menu that says "Show 50 messages per page," and highlight the number of messages you would like to display per page (the options are 50, 100, or 200 messages). If you would prefer to see all messages on a single scrolling page, select the option Show all messages in a scrolling list (with Preview Pane option).
3. In the upper left corner of the page, click Save Changes

When you return to your inbox, the selected number of messages will appear.