Go to the review tab on the top and tap the little page with the tiny globe in the top left corner. It will bring down a page that says language. Underneath that it says Proofing Tools and Word Count. Click on Word Count.
It is really easy, you just have to know where to look. With the document open that you want a word count for, just tap the “View” tab on the Ribbon, then tap “Word Count.” It will show you the number of pages, words, characters with spaces and characters without spaces.