Ideally none! You will always get a bit in there from people bringing it in on their feet, and from the wind blowing in in but you should try to keep it well-vacuumed so that the water stays cleaner and so that your filter will last longer.
concrete
12x12 pool above ground how much does it weigh
There is no filter at the bottom of a pool. Perhaps you are looking at the hydrostatic valve which is there in case your pool is emptied and Will make sure that the pool wont float in ground water.
If you don't have a hydrostatic valve on the bottom I would suggest at least a third. otherwise it may start floating in ground water if it is present.
Chlorine level in an 18' above ground pool should be between 2 and 4.
Turn the pool upside-down and knock on the bottom.
The pool will leak into the ground if a hole is eaten on the bottom.
The wrinkles are there to provide for liner shrinkage over the years.
Duck tape
You have no choice but to replace the liner
is there a hydrostatic valve in the bottom? These are designed to let ground water into the pool to avert the pool starting to float it way out of the ground. Once that hapens you are in big trouble.
Buy a pool patch