Check box used to select a mass of emails at once should have options
The check box at the top of the inbox that is used to select a mass of emails at once should include options for how many emails you want to select, such as 25, 50, 75, 100, etc.
The longer you scroll down, the more you can select at once. You can also delete all emails in a folder using the arrow next to it. You can delete up to 10k emails at a time in Yahoo Mail.