online email apps (the sort you use within a browser) only show 10 or 20 or whatever items at a time.
so you can only delete that many on one swoop.
they do however have a Settings, or the cog symbol, or something that allows you to change the number of displayed items per screen.
from memory Outlook.com (Hotmail) lets you show 100, I think Gmail can show 200 and some only allow a 50 items/screen limit.
so find the setting and change it to the maximum possible. but I doubt if you'll be able to show them all if you have 100's and 100's.
you may just have to empty the Trash more often or maybe there is a setting to skip the Trash altogether and always delete items permanently.