Skype for Business Online admin center users page limitations
For performance reasons the Skype for Business admin centerdisplays the first 1,000 users in your user list. If you have more than 1,000 users, use Filter and Search to find the specific people you're looking for.
Select the first 1,000 users—or everyone returned by your filter or search—by checking the box next to Display name at the top of the list.
You can change the number of users shown on each page with the control at the bottom of the list: