From the Sauron rules:
Thus in each Magic Leader Movement
Phase, including the one in which the Beast is
originally conjured, Sauron may use his magic to
give the Beast special capabilities. To do so is to
use his spell attempt for that Game-Turn.
Hidden away but it clarifies that you only get one spell attempt per game turn. At least that's how I read it.
I haven't mentioned it but I'm having a lot of fun looking up rules during your playthrough. I usually don't want to take the time when I'm playing so I just guess but I've wondered the above question for quite a while. Glad I finally found an answer. I'm still working on that catapult question...
Thanks for chasing this down! I'm working on some consolidated errata that I'll post, and I'm going to add this clarification to it.