When using Webmail you can manage how many messages are displayed per screen. Here is where you can edit those preferences in the respective webmail applications:
In the right hand corner select:
Settings > Mailbox View > Rows per page
Change that value to the value you would like to load at once. Default is 50.
Click the Settings Icon in the menu bar
Preferences > Mail > Mailbox Display > Messages per page
in the mailbox view. (Basic view only) change this value. Default is 30.