The reliable source is the recent letter of March 31, 2011
Transscript:
Dear Brothers:
We are also writing on behalf of Christian Congregation of Jehovah's Witnesses.
Thank you for your recent contributions to the:
*snip*
You will be interested to know that 791 Kingdom Ministry Schools were held within the United States branch between November 2010 and January 2011. Within just two months over 88,000 elders and 66,000 ministerial servants attended this school. The program was designed to deepen spirituality and help each carry out his work "in a fine manner."-1 Timothy 3:13.
Once again, we wish to express our thanks to you for your generous support of the worldwide work. May Jehovah continue to richly bless your hard work in his service.
Your brothers,
Watchtower B. <scribble>. Society
OF NEW YORK, INC.
Since all elders and ministerial servants were REQUIRED to go unless you were deadly sick I would say this count is accurate. The current qualifications for MS are (beyond the biblical): No recent child molesters, regular family study, regular meeting attendance, more than the national or congregational field service average - whichever is higher, no recent blemishes on your JW record. Most MS'es are going on to be elders now and most young people are either leaving or don't have the time or energy to put in many hours of service.