interesting fact

in early 2006 there were 881 general officers of all rank in all the branches of the US Armed Forces (active service only I believe).

assuming ~ 4 years mean time per grade, and an "up-or-out" policy, I estimate ~ 50-100 retirements per year.
About half of those should be Army and USMC officers.
Parsing the data more finely, something like 10 2-star Army generals retire each year. USMC is about 1/3 the size of the Army.

These numbers scale linearly with the change in assumptions about time in grade etc.