There is no specific answer, and you have to benchmark a realistic load to know.
Since you gave a number, though, that is pretty simple: you have two cores, and can deliver a PHP response in 0,01 seconds, so the math is:
2 (cores) * 1 (second) / 0,01 (seconds per response) = 200 pages per second.
You can then work out the longest response time you are willing to tolerate for a user, and the overhead for static assets, and do some math to work out a final number.
You can probably guestimate about 75 percent of that number as real user interaction for a simple site, or 50 percent for an AJAX heavy one, so between 100 and 150 users, more or less.
Don't forget to tune Apache to that load, and to make sure you have enough memory to serve it, though.