Well, There are no more hard and fast limits, unlike DOS, where you had to specify how many. The answer is, that it depends on resources, and what's consumed with each file. It also depends on resources required by the OS. One answer would be, a Multitude more under XP than under Vista, on the same hardware. The other answer is Depends on the response time reqired. If you measure response time of the computer in weeks, the number would be determined by your disk size :)
Bram van Kampen