Really, any history of the war that I've read, though, talks about high attrition rates in the combat units.
WW2 units were organized on a “tiangular” basis. A division was made up of 3 regiments, and a regimental combat team typically had 3 battalions, and so on. The prefered deployment scheme was “2 up, 1 back”. In extreme cases all RCT’s might be on-line, but then a sister division was probably contributing to the theater reserve.