How are you accessing your email? On my Droid Turbo 2 phone, using the Xfinity Connect app, there are boxes on the left of the messages in my Inbox that I can select to act on (copy, move, trash). There is a counter above with a down arrow that shows how many messages have been selected. I can tap the down arrow and the option to select all or unselect all is available. Using this I could send to trash all the messages in the Inbox. Then you have to select Empty Trash to permanently remove them.
I would like to empty my email inbox it is very full and has 30000 emails how can I empty it?
If you are using only the webmail interface and your browser, select the folder you want to empty, and to the right of it click on the "hamburger" icon. In the menu you see click on "Delete all messages". With the numbers you describe it might take a while to empty out the folder.