Discus always establish a pecking order within a group. Too small a group means the lesser fish in the pack receive the majority of the "bullying". The only way to "dilute" this is to increase the group size. I would say a minimum number should be six. I have a 55 gallon set up, fully planted and very well filtered, 2 30% water changes weekly using r/o. I stock 8 2.5-4" fish with no issues, in fact a pair have recently started spawning. I would say you could have a large group, up to 12, quite easily. One thing I always stock with my discus is an open water "calming" fish. A large group of cardinal tetra happily shoaling in open water seems to instill confidence in any shy discus. Corys are another useful addition, acting as a clean up crew as discus can be messy feeders.