1 adult discus per 10 gallons is pretty standard. I kept discus for years and am an active member on the simply discus forum. I would encourage you to buy all from the same source at the same time to prevent cross contamination. If you buy larger discus, bigger than 4.5”, their tolerance for nitrates is slightly higher than smaller discus. If you want to grow healthy thick discus from 2-3” juvies you really need to be doing 50% plus daily water changes until they hit 5” then you can back off. Try to keep your nitrates <10 where possible to avoid stress, hex and discus pimples