Channel Demonfire always fires 15 missiles to random targets afflicted by Immolate. The implication of this is that you can control exactly what target(s) to hit by keeping only one active Immolate. If your goal is to use Channel Demonfire to deal maximum AoE splash damage, it is usually best to operate off a single Immolate applied to a central target. For 2-target or council style encounters, applying individual Immolates is typically preferred due to their ember generation and damage.