Methinks that given that Spriggats and Shadowlings are slaves, and that, as per prior arguments, the spirits are likely to be tricked into coming (thus a very dodgy case of coming on their own accord, save for the single time when one fights a team of spirits who haven't realized that they died and simply show up every year to fight- those can be considered as mercenaries), thus can be considered as "slaves". Then, pretty much a mixture of merecnaries and slaves amongst humans, thus I would think that a slight majority of the participants are slaves. I would draw the parallel to the Colosseum, where throughout history most participants were slaves, rather than mercenaries.