I know some are saying that "6-12 isn't massive" but it's already tough with random players organising yourselves and making sure it's all happening correctly. Imagine everyone trying to discuss techniques and strategy with 11 other players, that's going to take some organisational skills, try adding even more. I do like the idea of a larger horde mode though and I imagine this will be something considered for destiny 2/comet/the taken king based on impressions of prison of elders.