Well, in Gears 1 we brought you 20 downs in one match of execution. Since Gears 2 is bigger, badder, and more badass, we're doing it not only with kills instead of downs, but also with 21 instead of 20.
So here it is, 21 kills in one match of execution. I had an awesome connection in this game, and could actually two-piece again (this was post-2piece tweak), so I went kind of crazy with it in the first 2 rounds before settling down.