i think 6 would be a good number. just get a filter that can handle a heavy bio-load and do maintenance on a regular. this way your water will stay pristine and your fish will be happy.
i have seen people do 6 in a 55 beautiful tanks....discus are so sweet. to further up this ? in smaller tank how many can be kept together ? like a 29 or is that wayyyy too small?
I wouldnt set them up in anything less than a 75. If you have to ask if you CAN put what ever kind of fish in a tank you probably shouldnt. if you want your Discus to look as brilliant as they should you need to meet their requirements.