Here's what just worked for me for gmail -
Go to your gmail account on a browser by typing gmail.com
Click the gear to the right and select "settings"
Navigate to "Forwarding and POP/IMAP" and scroll to the bottom where it says, "folder size limits"
Select "limit IMAP folders to contain no more than this many messages" with 1000 selected.
Click "save changes"
Refresh your email on your phone. It will drop to 1000.
Repeat for every gmail account ( if you have more than one)
Next, go into each gmail account on your phone and click "edit" then "select all" to "mark as read"
The number will drop, but not 0 out yet. Go back into your email, scroll as far down as you can go, go back out, refresh. The number should be 0 now.
Repeat for all additional gmail accounts.