Comments can be created by anyone. However, comment deletion can only be done by the person who had commented. Comments cannot be edited: Instead, delete the earlier comment and add a new one.
The number of comments on a card can be seen even before you open the comments dialog: Just hover the mouse on the [💬] icon in the main Kanban dashboard. You would see a small asterix at the beginning of the title of the card (in the dashboard) in case you had not read a comment someone may have placed.
NOTE: Creating a comment would consume some credits.