Twice that much may fall higher up at 4000 feet.

twice that much为主语，则much为不定代词。则that分析为程度副词修饰much, 而twice也分析为副词，修饰that。如果在much后加上不可数名词snow, 如下句：

Twice that much snow may fall higher up at 4000 feet.

则中心词为snow。twice为前位限定词（倍数词）, that仍为副词修饰 much，that much构成后位限定词。

Twice that many used cars had been sold.

作主语的中心词为cars，used为定语。twice为前位限定词，that many为后位限定词（that为副词修饰many）。