There are 3 summer sessions. I believe approximately 300 for each session.
As for number of summer seminar attendees that get appointments, there is no correlation whatsoever. You are not given any type of "Preference Points" whatsoever for attending summer seminar. When accepted for summer seminar, you haven't even started an application yet. SAT/ACT/GPA/etc... haven't even been factored in. And because only a small percentage of possible applicants can even attend summer seminar, those who weren't selected, or didn't even apply, do not have that held against them in any way, shape, or form.
Any numbers of summer seminar attendees that eventually do receive an appointment is purely coincidental. If anything; the only correlation that can be derived, is that those who DO apply for summer seminar happen to be more motivated individuals. Thus, they tend to be more motivated in the ensuing application process. And; having gone to summer seminar, they may be more motivated in their quest for an appointment, and put a lot more effort into the application and nomination process. But the actual attending of summer seminar does not give them any tangible advantage to receiving an appointment.