var mainCategorySimulatedResponse = {
  "d": {
    "results": [{
      "FileSystemObjectType": 0,
      "Id": 15,
      "ID": 15,
      "ContentTypeId": "0x010006DDBDD3BDC9914B92E911733F2A56E1",
      "Title": "A",
      "Modified": "2017-03-01T12:08:35Z",
      "Created": "2017-03-01T12:08:35Z",
      "AuthorId": 1,
      "EditorId": 1,
      "OData__UIVersionString": "1.0",
      "Attachments": false,
      "GUID": "c93d9db6-efd8-4b49-9024-f8cc30c0a48e"
    }, {
      "FileSystemObjectType": 0,
      "Id": 16,
      "ID": 16,
      "ContentTypeId": "0x010006DDBDD3BDC9914B92E911733F2A56E1",
      "Title": "B",
      "Modified": "2017-03-01T12:10:27Z",
      "Created": "2017-03-01T12:10:27Z",
      "AuthorId": 1,
      "EditorId": 1,
      "OData__UIVersionString": "1.0",
      "Attachments": false,
      "GUID": "a023853c-e967-4db8-8414-a7c390dcc09a"
    }]
  }
}

var pagesSimulatedResponse = {
  "d": {
    "results": [{
      "Title": "Home",
      "MainCategory": "A"
    }, {
      "Title": "ewrwe",
      "MainCategory": "A"
    }, {
      "Title": "tgtg",
      "MainCategory": "A"
    }, {
      "Title": "dsdfsfdfsdfsdfsdfsdsdfsd",
      "MainCategory": "A"
    }, {
      "Title": "jk",
      "MainCategory": "A"
    }, {
      "Title": "Cliff Curtis",
      "MainCategory": "B"
    }, {
      "Title": "Martin Henderson",
      "MainCategory": "B"
    }]
  }
}


$(document).ready(function() {

  function showMainCategoryData() {

    var mainCategories, pages;

    /*
    // Make nested ajax calls here, one for Main Categories, the other for Pages.
    $.ajax({
      url: url,
      ...
      success: function(data) {

        if (data.d.results.length > 0) {
          // store Main Categories data
          mainCategories = data.d.results;

          // call nested `Pages` ajax (only runs with Main Categories successful response)
          $.ajax({
            url: url,
            ...
            success: function(data) {
              if (data.d.results.length > 0) {
                // store Pages data
                pages = data.d.results;

                // display results
                displayResults();

              } else {
                console.log('Pages returned no data');
              }
            },
            error: function(data) {}
          });
        } else {
          console.log('MainCategories returned no data');
        }
      },
      error: function(data) {}
    }); 
    */

    function displayResults() {
      var results = "";
      var html = "<table class='mainCatTable'><th class='mainCatHdr'>Main Category</th><th class='mainCatHdr'>Pages</th>";

      //loop over Main Categories
      for (var catIdx = 0; catIdx < mainCategories.length; catIdx++) {

        var pageCount = 0;

        for (var pageIdx in pages) {
          if (pages[pageIdx].MainCategory == mainCategories[catIdx].Title)
            pageCount++;
        }

        html += "<tr><td class='padRight'>" + mainCategories[catIdx].GUID + "</td>" + "<td>" + pageCount + "</td></tr>";
      }

      $("#inserthere").append(html + "</table>");
    }

    /*************** emulation code only ***************/
    // emulate above ajax calls    
    function ajaxCatSuccess(data) {
      if (data.d.results.length > 0) {
        // store Main Categories data
        mainCategories = data.d.results;

        // emulate call to nested `Pages` ajax
        ajaxPageSuccess(pagesSimulatedResponse);

      } else {
        console.log('MainCategories returned no data');
      }
    }

    function ajaxPageSuccess(data) {
      if (data.d.results.length > 0) {
        // store Pages data
        pages = data.d.results;

        // display results
        displayResults();

      } else {
        console.log('Pages returned no data');
      }
    }

    // trigger first simulated ajax call

    ajaxCatSuccess(mainCategorySimulatedResponse)
    /************ end of emulate code ***************/

  }

  showMainCategoryData();

});

.mainCatTable {
  margin-left: 10%;
  width: 80%;
  border: 3px solid
}

.mainCatHdr {
  text-align: left
}

.padRight {
  padding-right: 10px;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="inserthere"></div>