Since it's 100$ first 24 h, thats 20x $40 you get discounted. So worth it.
However, I strongly recommend to contact them on
contact@xenforo.com with a pre-sales question for a bulk price purchase, while they don't offer one now, getting a 20x purchase in 1 go perhaps allows them to give you a custom paypal invoice with a pre-agreed total?
I would go with 20 licenses, if you need 20. And aren't buying them for reselling them more expensive to others
THere are still no details about transfer claus details in a EULA.