Completely depends on the weapon, I think Leigh.

I was ignoring something along the lines of a sniper rifle, and thinking of survival. I'm much more familiar with handguns than I am with rifles, but I think a full high cap clip for your weapon (or a full tube if it's built in like a shotgun), plus at least another clip or 2. If your weapon doesn't use a clip, then a bandolier is probably a decent option, but they do make so many styles of pouch, you could toss the ammo in one that fits it well.

I look at Hollywood for what it is. Entertainment. It's a lot more entertaining to hear a few hundred rounds buzz off and spray the area (with hardly any hits usually) than one shot. The exception maybe being the sniper movies. I pick weapons based on my own personal experience and knowledge. If some guy wants a fully auto AR15 or AK47 style weapon in a survival situation (at least not in a war zone!) then more power to him. I'll stick with a few well placed rounds and carry less equipment.

Anyway, I think at a bare minimum a dozen rounds for any given weapon is a nice start. I'd say depending on weight and mag capacity, maybe 45 rounds. That should suffice for any relatively minor survival situation as defense and hunting, but again, I'm just throwing numbers up in the air - it depends on the weapon, the situations and animals you might encounter, how effective the round is and how long you might be in said situation.