Welcome to Simply Discus, Mitch
5 is generally regarded as the minimum number of discus to be kept together so as to avoid, or at least minimize, any serious pecking order issues. (A mated pair being the exception).
However, keeping 5 discus along with 30 cardinals in a 65 gallon, particularly if it's planted, even lightly, is likely pushing the bio-load limitations, and making it difficult to maintain high water quality & conditions, which are a must for being successful at discus-keeping.
It would be best too if you did quite a bit more homework before getting started with discus - suggest you start with all of the Stickies (and The Video) located in the Discus Basics for Beginners Section.
Suggest you also consider getting a larger tank, say a 75 gal. or larger, and if you wish to start with a planted set-up, you're strongly urged to start with nothing smaller than 4"-5" sized near adult discus, obtained from a quality source for supplying healthy, well-shaped fish. See SD's Sponsors section for such suppliers.
Best of luck to you.