var sessions = [];
$('.rightDiv').each(function(index) {
    var session = $.trim($(this).text().slice(0, -1)).split("×");
    var sessionData = [];
    for (var i = 0; i < session.length; i++) {
        var ids = $(this).find('li').data('id');
        var s = {
            subjectOrder: i,
            subjectID: ids
        };
        sessionData.push(s);
    }
    var ses = {
        sessionNo: index,
        sessionData: sessionData
    };
    sessions.push(ses);
});

<ul id="form_builder_sortable_sample" class="sortable rightDiv session4 ui-sortable">
  <li class="draggable" data-id="1" name="Counting" >Counting<button onclick="deleteElement(event)" class="delbtn">×</button></li>
  <li class="draggable" data-id="2" name="Priorities" >Priorities" class="delbtn">×</button></li>
 </ul>

{"sessionNo":"0","sessionData":[{"subjectOrder":"0","subjectID":"1"},{"subjectOrder":"1","subjectID":"1"}]}

{"sessionNo":"0","sessionData":[{"subjectOrder":"0","subjectID":"1"},{"subjectOrder":"1","subjectID":"2"}]}

var sessionData = [];

$(this).find('li').each(function (index, elem) {
    sessionData.push({
        subjectOrder: index,
        subjectID: $(elem).data('id')
    });
})

var ids  = $(this).find('li').data('id');

var ids  = $(this).find('li').eq(i).data('id');