Deleting Multiple Emails on iPad Pro

You refer to an iPad Pro - yet the signature attached to your question suggests an iPad2 with iOS9?

If using an iPad Pro with iPadOS13.3.1 - and the native Mail App - at the top-right of the message list pane (the left pane if using Landscape mode), tap the Edit button - then touch Select All at top-left. Finally touch Delete at bottom-right of the message list.

Correct. If “filtered” (you’d not previously mentioned this), then no - there isn’t a “Select All” button. However, having touched Edit, you can just swipe down the list - rapidly selecting everything in the path of your finger - then touch Delete.