Last year, I applied to 7 schools. I received 7 secondaries, 5 interviews (Cornell, Columbia, Wash U, Tufts, U of AZ), rejected at 3, accepted at Tufts (too $$$ for me to go considering I didn't like it), and 1 waitlist (Wash U). I am reapplying this year with 24 schools. I was stupid not to give myself more options. I selected a much broader range of schools and have subsequently had an a$$-load of secondaries to fill out and pay for, but in the end, I'll be attending a school on my terms, not just because it was the only option.
And before you flame me about not taking my acceptance, and other vitriolic BS, I decided it wasn't worth $250K to go somewhere I didn't like.
Summary: It's worth the pain to apply to as many schools as you can - otherwise you run the risk of applying a second time, like me.