The rule of thumb among VCs is about 15 per year. Assuming $100+ million is considered a "big success". The cap for big success might be higher.

See more here: http://www.paulgraham.com/invtrend.html

Actually, this article here has some good quantitative data. It puts the number around 200. It looks like most of the companies are non-tech/outside the Valley though.

http://bostinno.streetwise.co/2013/05/14/only-200-startups-p...