The number of email messages displayed per page in a Yahoo email can be changed using the Mail Options. Click on the Options link available on the top of the right hand side. Select Mail Options from the menu list.
In the General Page, using the Messages/results per page field you can increase or decrease the number of email messages displayed per page.
The list of available values are
After selecting the required value, click on the Save Changes link available on Top of the General section. By this way you can change the display of email messages per page in Yahoo Mail.