By default, comments are disabled for announcements. When you create an announcement, you can allow comments on that announcement by selecting the Allow users to comment checkbox in the announcement options. Additionally, if an announcement allows users to comment, you can require students to post to the announcement before seeing other replies. The Allow users to comment option is persistent, meaning the option you select when creating or editing an announcement will carry over when you create a new announcement in the course.
You can also allow or disallow comments after an announcement is posted.
Note: If announcement comments are disabled for your course, comment options may not be available to you. You may be able to manage announcement replies in Course Settings. If this option is unavailable in Course Settings, it may have been locked by your Canvas administrator.