EB 2 refers to Employment Based Immigration category in 2nd preference. 2nd preference is allocated to intended immigration with Master's degree or higher education.
At the time of filing the application for green card, starting with request for labor certification, a date known as priority date is assigned. Priority date dictates when someone would eventually obtain the green card.
Only 140,000 visas (or adjustment of status applications) for employment based applicants can be granted in each year.
Subject to some adjustment for numbers flowing between categories, only 40,040 visas can be issued for EB2. When an immigrant enters the U.S. as an EB2, with a spouse and two children, four visa numbers are used - not one. The average employment based immigrant family will use between two and three visa numbers. That means less than 70,000 families will enter the U.S. each year as employment based applicants - and fewer than 20,000 families based with EB2 preference category.
Here is a great article by a fellow attorney that discusses the problem very well. Again, read the article here by Attorney Jay Solomon.