You can delete up to 100 emails at a time in Full-featured Mail or 1 page at a time in Basic Mail. Put a check in the box at the top left of the message list to select all and then hit Delete: https://help.yahoo.com/kb/sln28054.html
Not my mail. It doesn't work that way at all. I have Yahoo Mail Pro. I pay for it. I put a check in that box, and it deletes 40. Next time I check the box, it deletes 100, then next time 200, and then 450. I have to refresh the page to go back to 40. I don't ever want to delete over 100.