as many as 20 days正确。as many as和as much as都可以修饰可数名词，但要注意以下区别：

1. as many as+数词+“个体”复数名词。例如：

The great fire burned down as many as 20 buildings. 那场大火烧毁的建筑物达20栋之多。

As many as 15 people were killed in the school shooting. 在这次校园枪击案中的死亡人数多达15人。

2. as much as +数词+“单位”复数名词。例如：

The huge stone weighs as much as 15 tons. 这块巨石重达15吨。

He was paid as much as 100 pounds for a single appearance. 他一次出场就被付给一百英镑之多。