A step in a user journey can have unlimited amount of comments attached to it.
To add a comment you first need to enter the journey edit mode. (you need to have edit rights), by clicking on the 'Edit Journey' button.
Select the step in the user journey you want to add a comment to. In the dialog widow click on the speech bubble icon to add your comment.
Write your comment and if able connect a mood to the comment. If you choose not to include a comment it'll be classified as a general comment connected to your profile and not as an persona observation.
When you're done you can choose to add more comments / journey steps etc. or save the journey by clicking Save Journey. Please noter that the journey and your new comments are not saved until you click Save Journey.