How do you spell ahhhhh?

The spelling of the exclamation (onomatopoeia) is "ah" or longer "ahh."

More than 2 H's indicates an extension of the sound that would normally only be used in dialogue.

The term can indicate recognition (Ah!), or a sigh of comfort (Ahh...), or possibly fear (Ahh! or Yahh!) which would have to be indicated by the context.