Many times, your professor will request that you share a response that's a certain number of words: 200, 250, and so on.
It's good practice to compose your response outside of Canvas. Write it in Microsoft Word so that you can save it often. If it messes up when you post, you'll have a back-up. Plus, if you use Microsoft Word to write your post, you'll be able to use the built-in word counter.
If you are using the Canvas discussion page to compose your text, you'll see a word counter as you're typing at the bottom of the text box. This can give you an idea of how long a reply may be, for example.