When using Webmail you can manage how many messages you are able to view per screen. Here is where you edit those preferences in the respective webmail applications:
In the right hand corner select: Settings > Mailbox View > Rows per page (the default is 50) Change that value to the value you would like to load at once.
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 (the default is 30)