Seven.
Seven souls to break the barrier, and the Underground will go empty.
When Undyne hands him the last soul, Asgore thanks her quietly and takes it into his own hands and into his body and into his body and into his body and she hands him each of the souls, each a different color as it passes through his hands and into his body into his body and goes into his body until there are seven souls, seven souls fused with his own, and he is not Asgore anymore.
His body twists and malforms, he is he is he is a g o d and he is not Asgore. His fangs are long, his claws are sharp, and he throws his body at the barrier, but it does not break. And he throws his body at the barrier, but it does not break. And he throws his body at the barrier, but it does not break,
He was lied to, he realizes too late.
But it is too late, and he can feel the burning hatred of the human souls' wrath, their poisonous venomous bitterness; it would eat him alive if despair didn't get to him first. He is lost he is gone he is left behind at the barrier while his body bounds away, consumed by hate wrath fury vengeance. His body is gone and it had torn through Undyne and he can feel nothing. His own claws reduce his people to dust, and he is far far far far far away and he thinks it is better this way there is no hope it is better this way, and when Toriel larger than life comes and tells him she was right all along and he a scoundrel, a beast, a hateful monster she regrets meeting, he says yes, and when she puts him down, he thanks her as his mouth turns to dust.
And he is left alone in the darkness in the silence in the dust, and it is all he can see and all he can taste and he cannot leave or move or exist because he is dead and all is lost.
It feels like an eternity has passed when he wakes up, his head pounding.