Only one. I figure it's a better deal to spend $120 replacing it in three years, if necessary, than to pay for another one off the bat that'll also be dead by then. And as others have said, that's cash that could be put toward drive or memory upgrades :^)
I agree. I've often thought about getting a 2nd battery, but due to the high price i'm not planning on getting one until after 2-3 years. Then i'll just keep the old one around until it fails to hold a charge. If I did a lot of traveling or worked while on the road I would get extra batteries, but most of the time my MacBook just sits on a stand tethered to AC power. The few times i've travelled (2 road trips), i've never had a need to use the laptop outside of a hotel, so i've never been somewhere that I actually had to rely on battery power.