How many survival uses are there for 2 litre plastic bottles?

Depends on how you count and how you define "survival".

I am a big fan of plastic PET bottles. Now these come in two flavors: One type is wrinkled to make it easier to cram them together so they don't take up so much space in the trash or in the recycling bin. Bottled water is sold on these kind of PET bottles in many countries. The other kind is not wrinkled, and MUCH stronger, which make them more versatile. Carbonated beverages are often sold on this kind of container, and they are also used for non-carbonated drinks in some countries.

PET of the non-wrinkled kind is a fantastic material for all sorts of makeshift improvising. You cut it with a knife to the desired shape and then fold it so it fits. Finish with duct tape to hold it together. Voila, instant knife sheat. If you know how strong a curved shaped piece of PET bottle is then you'll see lots of uses.

I just used two 1.5 liters carbonated soda bottles to guard a paraffin lamp glass that someone will transport for me. Wrap the lamp glass in bubble wrap, cut off the top of each bottle, insert the lamp glass into one bottle and put the other bottle on the other end of the lamp glass. Tape together and we're done. Lamp glass is fragile stuff, but now it is fairly bump and squeeze resistant. Not good enough for airline transport, but good enough for being in the trunk of my friends car.

Now Dweste clearly said "plastic", so the discussion should not be restricted to PET. Generic plastic containers does have some of the merits of PET, but most plastics aren't as strong as PET.

Any container can be filled to hold anything that you can fit into the opening. Including those small tools and screws that you absolutely want to make sure you don't loose.

To make containers for slightly larger objects cut off the top part of two bottles, insert whatever into one of the bottles and insert the other bottle on top of that. If you can find bottles of slightly different sizes that would be helpful. This is what I use to protect the burner head of my optimus-00 stove. The flame ring gets very fragile due to the high heat, but now it is well protected during transport. For bottles of the same size just make a vertical slice on the bottle that goes inside the other bottle.

Fill it with sand or gravel (and possibly top it off with water) to make a weight to hold something down, such as anchoring the last flapping corner of a tarp.

For digging into loose material you can make a small shovel by cutting away about half of the bottle. Not good for hard packed dirt, though. I've cleaned plenty of ash using a small make-shift shovel made from a PET bottle.

Buoyancy devices - filled with air and taped together, or inserted into your jacket.

Or fill them with drinking water so you don't die of thirst....

three hundred and forty-two

You can make a portable column filter with one.

http://www.youtube.com/watch?v=QrWPErQ0m38

UV purifier....clear bottles......aeriate non potable water by shaking...hang in exposed sunlight 8+ hours

minnow trap....2 liter... cut off top, reverse cone section..add entrail for bait

micro scale solar desalinator...2 liter... cut off top cone section with non leaking cap installed..small amount of brackish or salt water in bottom of bottle...small catch cup (weighted)centered in bottle (a funnel that makes a contact seal with the rim of the catch cup helps keep the condensate from re vaporizing)...reverse cone section to bottle...add cool water to the reversed cone section to improve condensation rate...use pre warmed top section water for brackish source...make a dozen or so

chandelier...water filled clear bottles to focus/project small light source

SODIS water disinfection.

http://en.wikipedia.org/wiki/Solar_water_disinfection

Or strung together with paracord using constrictor knots.

Consider both intact and cut apart, filled and unfilled, bottles as construction material; think tinkertoys or legos; cinderblock, tiles, glassblock....

Consider cut apart bottle material as light, self attaching, shielding or armor, braces, gauntlets, replacement footwear, chaps, masks, material roll holders / clips ....

The SODIS method only works if you're within 30 degrees of the equator, I think.

Cut the tops and bottoms off several, make one full-length slit in the side, and line them up to form a splint for a broken arm.

Cut off the top half and use the bottom as a cup or scoop, and the top as a funnel... if you need a funnel.

All of this assumes the bottles are either 'fresh' or haven't been sitting in sunlight for very long, or they'll just shatter.

Sue

Wind screen for small stoves and candles, keeping the plastic carefully away from high heat.

Ala Mr. Samuel Thayer, create right size hole to fit over cattail heads which are then "knocked" to collect highly nutitious pollen inside a clean, dry container.

Filled bottles can act as weights to close traps and trap doors after the traps are triggered.

Sun- or fire-warmed water-filled bottles as the classic hot water bottle.

Rigged with lines, hoses, or tarp "gutters" running to their mouths: rain water collectors.