Formed by Army Order 414 of 26th October 1915, the Machine Gun Corps (MGC) comprised of three branches.
In addition to this, the Heavy Section Machine Gun Corps was formed on 14 July 1916 by Army Order 253 and it was this unit which was equipped with the first Tanks. Army Order 239 transferred this unit to the Tanks Corps.
Also, whereas Infantry and Cavalry of the Line units formed the respective branches of the MGC, the Guards Regiments formed their own Guards MG Battalion which later became the Guards MG Regiment.
The Machine Gun Corps, and all it's branches, were disbanded in 1922.
The strength of the Machine Gun Corps was recorded from January, 1915, until March, 1920, on a month-by-month basis.
At its peak, in December, 1918, it consisted of 9,707 officers and 149,089 other ranks - a total of 158,796 all ranks. A more detailed analysis is available on the MGC strength page.
For more information on the Machine Gun Corps, you may find the following books of interest.