I thought that it was more (around 1000), and I have heard of various hacks to increase it, but I really don't know as I don't use the CR parameter screens (we have an application that uses wizards to gather the parameters for our stored procs)
With that said, have you run the same query in the database and seen the results? For the dynamic fields, it won't show all combinations as some combinations will result in an empty report, and so CR doesn't display that option.
HTH