I'm not 100% sure whether you are just wanting to count the number of strings in the myStrings array, or just have the ability to add a count, but given your Plunker, you could do the following:
To simply add two variables, update the following line:
$scope.stringIwantToBeCompiled = 'I want to count to 4 via angular: ' + (2+2);
If you wanted to show the count of the number of strings, swap the order of your scope variable declarations and show the myStrings length
$scope.myStrings = ['I am a string', 'I am another string', 'more strings!'];
$scope.stringIwantToBeCompiled = 'I want to count to 4 via angular: ' + $scope.myStrings.length;
Counting the strings will only give you 3, of course, because there are only 3 strings in the array.
Does that solve it for you?
UPDATE
OK - So I think what you want is the count in the string with an ng-click to correspond to the count correct?
If so, then the following on your ng-repeat would do it...
<p><a href="" ng-click="someMethod($index)">{{string}} {{$index}}</a> </p>
Using $index gives you the index of the repeating item. You can always add 1 to the $index to make it 1-based instead of zero based:
<p><a href="" ng-click="someMethod($index)">{{string}} {{$index + 1}}</a> </p>