I can't tell you how many times i've asked this question. After a while i've come up with two solutions:
1) Use and Evox trainer, or the Xored mini launcher to run a trainer from any dashboard. There is a trainer which enables unlimited grenades.
2) Use an app called Halo2USE made by TfAv1228. With this app you can open the Halo 2 XBE file, and enable unlimited grenades. here is a pic
This probably isn't what you're looking for. I know it would be cool to set it to a specific number. Either way its mad fun. If you use tfAv1228's app remember it only works with the 1.0 version XBE.