Do artists discard of most of their work, or do they keep or sell it all?

A good self critical artist  will complete a painting/drawing/work etc.,  Put it away for say, six months.  Then come back and look at it , with a critical eye.  From that point he/she will decide which to keep and which to destroy.   About three quarters of the work should be destroyed, because he will see the faults and how his current work has improved. .  

yes, they discard most of it, unless they are rich and have plenty of storage space. Most of us don't have a dedicated room with a walk-in closet to store our things.

I taught myself to paint over many years , its for the love of it friends have had a few but most are not for sale and they are all over the place in my home , probably 30 are hanging in houses as far away from England as Australia , its not in my mind to make money and i paint what i want there is no price for me 

Depends on the artist.  Some have a natural talent where they can begin painting and have masterpieces from day 1.  Others go through thousands of paintings and never get anything that is worth keeping or selling.

It is obvious that most painters abandon paintings at times.  Many of the best known masterpieces of day are painted over pictures the artist did not like or finish.  X-ray shows several changes that Leonardo made to the Mona Lisa, for example.  And evidence that there was an earlier painting on the piece of wood (it is painting on wood not canvas).