I've heard rumors that in Halo 3:ODST the player will be able to carry 4 of each kind of grenade for a total of 16 grenades. In this video here though, it proves that the player will only be able to carry 3 of each kind of grenade.
If you pause the video at 4:21 you will see that the player has 2 plasma grenades already.
The player had just previously killed a grunt, and when the grunt died, he dropped 2 plasma grenades. Now if the player is able to hold four grenades of each kind of grenade, when he walks over the dead grunt, he should pick up both of the grenades to have a total of 4.
Now if you play the video for a couple more seconds you will see that the player walks over the dead grunt. When he walks over it though, he only picks up 1 of the grenades for a grand total of 3 grenades.
This proves that we will only be able to hold 3 grenades of each kind for a grand total of 12 grenades, which is still more than we've been able to have up to this point.
If somebody could supply a video that shows a player holding 4 grenades of one kind of grenade, like 4 plasmas or 4 frags, it would be greatly appreciated