Comments Limitations

You can have one active discussion on most fields. However, the JSON Rich Text Editor (RTE) supports multiple active discussions.
Each content block, text, or image within the JSON RTE can have one active discussion.
A maximum of 100 active discussions are allowed per entry per locale.
Each discussion can contain up to 1,000 comments.
The maximum message length allowed for each comment is 500 characters.
You can mention a maximum of 20 users and 20 roles in a single comment.
You cannot export entry discussions or comments while exporting the entries.
You cannot view comments for unlocalized or older entry versions.
Comments can only be added to subfields of Group and Global Fields. Similarly, for Modular Blocks, comments can only be added to fields within each block and not to the parent field.

More articles in "Manage Entry Comments"