A bunch. Which is more than a few but less than a ton.
You're asking a bit of a trick question since the the resource demands will be directly related to the content being served. The only reasonably accurate way to arrive at a number is to monitor server performance as you add clients to extrapolate what your maximum might be.
Enterprise Network Engineer :: Hosting Hobbyist :: Master of Procrastination
"The really cool thing about facts is they remain true regardless of who states them."