To find the number of grunts killed so far click on "Online Stats", not on its drop down. Just underneath the "enter gamertag Find your Stats!" bit you should see something that looks like this: < | | | | | >
Press the arrows until you see a picture of a brute. On the drop down menu that has now appeared next to the brute select grunt infantry, add the number killed all time to the number killed all time for grunt specialists and you have the total number of grunts ever killed. When I last checked, about 5 mins ago, it was 3,686,887,719.