Canvas Speedgrader does not have a Word Count feature, though it is under development according to this thread. Before this feature becomes available, you can get your word count in either of these two ways. This may be a fairly minor issue, but it can be annoying if you keep having to copy text into a word processor for word count while grading.
Display word count with an Browser extension
If you use Chrome or Firefox, install an extension called “Word Count Tool”. I could not find a similar tool in Safari yet, but please share if you find one.
Once installed, highlight any text (or the entire paper), right click, and then click on “Word Count Tool”, you will be able to see the detailed Word Count information, like this:
Display word count with Turnitin
If you enable Turnitin for student assignment, Turnitin allows you to see the Word count for the paper fairly easily.
Click on the percentage that represents matching, such as “8%” below.
Click on “Text-Only Report” at the bottom right corner.
Now you should be able to see the word count for the submission.