255 grenades
Change your weapon to the grenades and hold Z until the throw meter is filled. When the grenade is being thrown, hold Z again until the meter is full. Repeat this until you are on the last grenade. When it is thrown, you will have 255 grenades.

Level select
Enter "VZRFTMQ2G8SQ" as a password for the recruit difficulty setting. Enter "FZJFTMR2G8RQ" as a password for the veteran difficulty setting.