You’re going to want to size your dehu(s) for the amount of water you’re needing to remove from the air. 70 gallons of coco is probably taking quite a bit of water, even discounting what gets drained to waste. Almost all of that will end up as water vapor that you’re going to have to deal with, either through ventilation or dehumidification.
For example, if you’re watering with 12 gallons and collecting 2 gallons of run-off you’ll need to deal with 10 gallons of water vapor per day, or 80 pints. As RH goes down with better control you may find watering needs go up as the evap rate increases, so oversize your dehu. (2) 50 pint dehus is my guesstimate in this situation, although I’d start with one and see what happens.
Rating standards for dehus changed recently. Used to be they were rated for how much water they took in 24 hours at 80F / 60%. Now it’s tested at 65F / 60%. Not all dehu mfg have changed their ratings yet. The typical sizes available at the hardware store were 35, 50, and 70 pints. Those same dehus are now rated 20, 30, and 50 pints.
One last bit of advice - make sure you have a plan for draining the condensate. My advice would be for gravity draining into a floor drain, as the internal pumps in these units are nor reliable in the long term. If no floor drain is available get a separate condensate pump. You will never empty the bin often enough to be effective.