Which version of Yahoo mail are you using. If it is the new version you will not be able to set it to a certain number because the page is scrollable. If it is the old version then on the right corner to next your username there is a settings icon. Right click that icon and click mail options and in the general tab you can set the number of mails in a page.
Which version of Yahoo mail are you using. If it is the new version you will not be able to set it to a certain number because the page is scrollable. If it is the old version then on the right corner to next your username there is a settings icon. Right click that icon and click mail options and in the general tab you can set the number of mails in a page.