正确的句子应该是:

1. Let's try to speak as much English  as possible. 让我们尽可能多说英语。 

2. Please write as many sentences  as possible. 请写出尽可能多的句子。 

as much as possible 有两种用法：1.  Let's try to speak English as much as possible. 2.  Let's try to speak as much English as possible.  as much as possible 可以像副词那样独立使用，类似的如：as soon as possible. 

as many... as possible 这样用多见：Please write as many sentences as possible. as many as possible，一般不能独立像副词那样用，而做宾语或主语的比较多见, 如：1. They kill as many as possible.  2. He was determined that as many as possible would be killed as quickly as possible. 3. Individual researchers might have personal preferences for some data generation techniques, some kinds of questions, or some forms of data, but it is a very good plan to understand as many as possible.

解答如下 供网友参考

【一】让我们尽可能多说英语。 

【1】 Let's try to speak English as much as possible. 【状语】

【2】 Let's try to speak  as much English as possible. 【宾语】

备注：让以上2种表达结构都是正确的。English为不可数名词，用much。

【二】请写出尽可能多的句子。 

【1】Please write sentences as many  as possible. 【状语】

【2】Please write as many sentences  as possible. 【宾语】

备注：让以上2种表达结构都是正确的。sentence为可数名词，用many。

以下是2种表达结构讲解供网友参考

【1】 Let's try to speak English as much as possible.

备注：以上“as much as possible”充当副词语法功能，作状语，修饰动词，即修饰不定式to speak English

to speak【不定式】 English【宾语】 as much as possible【状语】.

【2】Let's try to speak as much English as possible. 

备注：以上“as much English as possible”充当名词语法功能，作动词宾语，即作不定式to speak宾语，

to speak【不定式】 as much English as possible【宾语】 

网友搞清楚以上as much as possible/  as much+不可数名词+ as possible 和as many as possible/  as many+不可数名词+as possible在句中充当“副词语法功能=作状语” 和在句中充当“名词语法功能=作宾语”后，再也不会弄糊涂了。并正确熟练使用以上两个不同表达结构。