The spell must both be cast and deal hp damage
The ghoul sorcerer bloodline arcana says that
Whenever you cast a spell of the necromancy school that deals hit point damage, you are healed of 1 hit point per spell level.
That means for each necromancy spell both that's cast and that deals damage, the sorcerer heals 1 hp per spell level. And it means that the effect occurs only once upon each iteration of both casting the spell and dealing hp damage with that spell.
For example, the first time the sorcerer deals at least 1 hp of damage with the spell chill touch after the spell's cast, the sorcerer heals 1 hp of damage due to his bloodline arcana, but on the second and successive touches with the same chill touch spell, the sorcerer heals no further hp damage.
For example, when the sorcerer casts wail of the banshee, if he deals with that spell at least 1 hp of damage, he heals 9 hp of damage due to his bloodline arcana, but he won't heal more than that 9 hp of damage, no matter how many creatures the wail damages.