ShowCommentCountOnList.json

{
  "$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
  "elmType": "button",
  "customRowAction": {
    "action": "defaultClick"
  },
  "attributes": {
    "class": "ms-fontColor-themePrimary ms-fontSize-m"
  },
  "style": {
    "border": "none",
    "background-color": "transparent"
  },
  "children": [
    {
      "elmType": "span",
      "style": {
        "display": "=if([$_CommentCount] == '' , 'none' , 'block')"
      },
      "attributes": {
        "iconName": "Comment"
      }
    },
    {
      "elmType": "span",
      "txtContent": "=if([$_CommentCount] == '' , '0' , [$_CommentCount])",
      "style": {
        "margin-left": "10px"
      }
    }
  ]
}