Yeah if you have enough filtration you could put 8-10 in there if you really wanted to. They school together really well.
I'd probably start off with 3 and work my way up to 5-6.
Basically the most important things with keeping them is a stable acidic PH and a constant high temperature with crystal clear water quality. Because my current PH is always 7.4 (same with tap) I would like to avoid using chemicals to buff the ph down everytime I do water changes, so I'll use a fair amount of driftwood. Would doing 20% water changes weekly with 7.4 tap water into acidic cause chaos with discus, think its worth buffing buckets of water down before adding to tank?
I'm amazed at how well my fluval 305 filtrates, my water quality is amazing and all my plants and fish are thriving currently.
Got two decent size pieces of Malaysian driftwood from the pet shop, soaked it in a bucket for a couple of days. Didn't release any tannins at all, did lower my 7.2 tap ph to 6.0 in the bucket. Ive had it in my tank for about a day and a half and it still hasn't released any tannins. Odd enough it hasn't changed my 7.5 tank ph at all.