nonsense. you can easily get away with around 12-15 pygos @ 6-7". filtration would be an issue, you'd definitely need some heavy duty stuff. but if you can provide adequate filtration, i'd shoot for the double digits.
A dozen for a good few years, imo, monitor water closely for PH and Nitrates to determine how many water changes weekly you need to perform or if a buffer needs to be added. Once p's reach 12" plus you may need to sell a few.