3-5...if they're interested, they'll contact you and ask for more.
The advice about really wowing them early on is dead on. I worked as Music Director for a college radio station for a few years in a small college town. You have no idea of how many Demos I received from all over the country, not to mention international acts. As an artist I wanted to give everything a fair listen, but it just isn't possible. some CDs got 10 seconds max to capture my attention.
An A&R at a big label...imagine how many demos they get. I don't mean to discourage you, just let you know what you're up against. Also, focus on some sort of press kit...to set yourself apart and outline your vision. Def. won't hurt....sometimes reading a good press kit made me want to like a demo and give it a chance...just FyI
EDIT: PUT contact info. directly on disc!!! More important than you think!