For the regular version you can only delete 200 mails at one time. Click the gear symbol, then Options > Mail Options > General page. Under the heading ‘Messages settings’, check the button for the number of messages per page. Click which option number you want. THEN click ‘Save changes’ in the upper left corner. Click ‘Select all’ or check the small box beside the From tab, (above the other Inbox check boxes), then ‘Delete’, as usual, or use the keyboard shortcut Ctrl + A, then Delete.
The older regular version of mail used to have the option of ‘In a scrolling list’ so you could delete all messages at once – hopefully this option will be re-instated.
For the Classic Mail version, you can only delete one page at a time, but you can assign more mail to a page. Click Options > Mail Options > General, and in Messages/results per page, click the arrow beside the number. The maximum is 500 per page. Remember to save the changes. As usual, to delete many at once, click ‘Check All’ (or the small box above the Inbox list), then ‘Delete’ (or Ctrl +A, then Delete).
Answers is a Jungle