A step in a user journey can have unlimited amount of comments attached to it.
To delete a comment you first need to enter the journey edit mode. (you need to have edit rights to activate edit mode), by clicking on the 'Edit Journey' button.
Once in edit mode you click on the step that has the comment you want to delete. A popup appears next to the step with all the comments listed below the step description. Click on the 'X' icon to the right of the comment you want to delete.
Finally, when you're done deleting, click on the 'Save Journey' button to save your changes.