two confusions on Idiom part

[JhanasC520]

Dear Expert,

I got quite confused about the usage of "double" and "as many as". previously, I thought this two usage is similar until I read two sentences on Manhanttan Idiom part.

1 Naomi wrote ten letters, double the number that Sara wrote.
2 We have 10 apples, about as many as we picked yesterday.

Could you please explain, why in sentence 1, double should be followed by the number. why the sentence can not be rewrited as Naomi wrote ten letters, double those Sara wrote.

Please help clarify the different usage beween double and as many as

[RonPurewal]

you can't "double" a bunch of letters. you can only "double" numerical quantities.