test-group-header.json

{
  "$schema": "https://developer.microsoft.com/json-schemas/sp/v2/row-formatting.schema.json",
  "groupProps": {
    "headerFormatter": {
      "elmType": "div",
      "style": {
        "flex-direction": "row",
        "border-radius": "5px",
        "padding": "10px",
        "margin": "3px"
      },
      "attributes": {
        "class": "ms-bgColor-themeLighter"
      },
      "children": [
        {
          "elmType": "div",
          "txtContent": "=@group.fieldData+@group.fieldData"
        },
        {
          "elmType": "div",
          "style": {
            "margin-left": "5px"
          },
          "txtContent": "='('+@group.count+')'"
        }
      ]
    }
  }
}