Assuming an average altitude of 1000 miles, the area covered by this space junk is about 255 million square miles. With 20,000 pieces of space junk, that works out to one piece of space junk for every 12,700 square miles. That's about one piece of junk for every area the size of the state of Maryland.
But the orbiting junk travels fast (5 miles/sec) so it sweeps out large volumes per unit time and would impact other satellites at high relative velocity in the miles/second range, creating more debris in a cascading effect / chain reaction.