Navy Aegis Ballistic Missile Defense (BMD) Program: Background and Issues for Congress [October 13, 2017] [open pdf - 1MB]
"The Aegis ballistic missile defense (BMD) program, which is carried out by the Missile Defense Agency (MDA) and the Navy, gives Navy Aegis cruisers and destroyers a capability for conducting BMD operations. Under the FY2018 budget submission, the number of BMD-capable Aegis ships is scheduled to be 36 at the end of FY2018 and 51 at the end of FY2022."
CRS Report for Congress, RL33745
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html