as with any web and database process, the number of requests is subjective and may actually rely on the design of the site as much as the database used. been there, done that. a web site can be optimized to use the data effectively and limit the calls to the database.
requests over what period of time?
The specifications limit that the maximum number of simultaneous connections is 2000. FileMaker has tested it at 200 simultaneous connections. That being said though, connections isn't concurrent users, it's the number of people who would make a database call at the exact same time.