it作形式主语,指代实际的主语从句how much this helped.
如果把主语从句how much it helped放在主语的位置,那这个句子主语太长、谓语部分(is amazing)太短,头重脚轻了,为了使句子平衡,就用it作了形式主语,把实际的主语放在后面了.
这个结构可以总结为：
It is +形容词/名词 +主语从句/不定式/动名词.
eg:It is necassary that we should learn English.
It is necessary (for us) to learn English.
It is no use arguing with him.