After doing some research, their 2007 student body consisted of ~15 students coming in. This would suggest that they would accept at least 20 students imo.
If MIT is an accurate consideration, their website claims that they accept twice as many students as there are space each year. "We receive over 400 applications per year, and of these, we admit approximately 40 students and enroll between 10 and 20 students."
http://bcs.mit.edu/academics/grad_howtoapplyfaqs.html
If MIT over-accepts that much, than I can imagine that Harvard might hedge their bets a little as well.