$('.addCustomEmbed').on('click',function(){
  var str= $('<div id="gecici">').append($('ul.part_list').find("a[data-toggle=tab]").clone()).html();
  var pattern = /#tabkaynak-([0-9]+)/gi;
  var myArray = str.match(pattern);
  var newArray=[];
  $.each(myArray,function(i,v){ newArray.push(v.split('-')[1])});
  var newNum= Number(newArray[newArray.length-1])+1;
  $('.part_list li').removeClass('active');
  $('ul.part_list').append('<li class="active"><a data-toggle="tab" href="#tabkaynak-'+newNum+'">Kaynak #'+newNum+'</a></li>');
  return false;
});

<button class="btn btn-primary addCustomEmbed" data-type="normal"><span class="icomoon-icon-download-2 white"></span> Kaynak Ekle</button>
<ul class="nav nav-tabs part_list">
<li class="active"><a href="#tab720p" data-toggle="tab">720p</a></li>
<li ><a href="#tabnosubtitle" data-toggle="tab">nosubtitle</a></li>
<li ><a href="#tabtr" data-toggle="tab">tr</a></li>
<li ><a href="#tabsubtitle" data-toggle="tab">subtitle</a></li>
</ul>
<div id="gizli"></div>

$('.addCustomEmbed').on('click',function(){
  var str= $('<div id="gecici">').append($('ul.part_list').find("a[data-toggle=tab]").clone()).html();
  var pattern = /#tabkaynak-([0-9]+)/gi;
  var myArray = str.match(pattern);
    if(myArray==null) {var myArray = 1; }
//yukardaki işe yaramadı.
  var newArray=[];
  $.each(myArray,function(i,v){ newArray.push(v.split('-')[1])});
  var newNum= Number(newArray[newArray.length-1])+1;
  $('.part_list li').removeClass('active');
  $('ul.part_list').append('<li class="active"><a data-toggle="tab" href="#tabkaynak-'+newNum+'">Kaynak #'+newNum+'</a></li>');
  return false;
});