I believe that 2 midshipmen will be going to OCS after graduation this year. I don't know if more applied and didn't get a spot, but I got that feeling that just 2 students sought out the Marine Corps.
The number of mids going to the Marines varies from year to year, so I don't think there is a set number of spots. It's really more about whether the student can handle the school work at KP plus the extra fitness prep and training. The academics here are brutal and culminate with the Coast Guard licensing exam as one of the conditions of graduation. No other academy has a license exam (the Coast Guard grads do NOT get a Coast Guard license.) An engineering degree in 3 years (you are out at sea for one of the four years) is not for the faint of heart and probably influences how many people can put up with the extra training it takes to qualify for the Marine Corps. But some do make it.