There's no way that number can be accurate.
According to Georgetown University’s Center for Research in the Apostolate, in 2009 there were 59,601 total sisters in the United States. Clearly, not all of them share the assertions of the Network statement.
In fact, as Mary DeTurris Poust points out in a previous post here, the Council of Major Superiors of Women Religious, which represents about 10,000 sisters, issued a statement supporting the bishops’ analysis.
Something doesn't add up.
UPDATE: Sister Mary Ann Walsh, director of communications for the U.S. bishops' conference, just issued a statement making the same point. Here it is:
Washington—A recent letter from Network, a social justice lobby of sisters, grossly overstated whom they represent in a letter to Congress that was also released to media.
Network’s letter, about health care reform, was signed by a few dozen people, and despite what Network said, they do not come anywhere near representing 59,000 American sisters.
The letter had 55 signatories, some individuals, some groups of three to five persons. One endorser signed twice.
There are 793 religious communities in the United States.
The math is clear. Network is far off the mark.
Sister Mary Ann Walsh