Yup, definitely 14 guards total in the Tank mission. I killed them ALL with Battle Snake and it recorded 14 kills total at the end. My final shot was a RPG in the face of the Tank Commander. So satisfying!
Also, I just saw something really cool while selecting Battle Snake before the mission. In the setup screen where you select the suit, I noticed the sections are actually Character, Suit, Weapons, Items. The scrolling text at the bottom of the screen specifically says: In this screen, you select the weapons, items and costume to take into the mission, for this demo, your weapons and items will be automatically selected based on the suit you select.
Full customization confirmed! The difference between the suits are that each suit determines your a) camo type/index, b) the amount of weapons and items you can take into the field. You will notice that different suits in the demo have different amount of weapons and items, that is what I believe will be the max number of free slots you have when using that suit in the final game. Also, having a Character section there, also pretty much confirms there will be playable characters other than Snake.