"A sounder of boars, wild [12+]"
http://users.tinyonline.co.uk/gswithenbank/collnoun.htm
So, 12 or more Wild Boards per summon? Sounds right. "Sounder of Boars"? Well, that's a bit strange, but hey, it's grammar.
P.S. "Sounder of boards" was even stranger. I hate typoes like that.