If you see twice as many X's, or [i]half as many X's, or three times this/that number of X's, etc. -- just treat it in the same way you'd treat "two X's" or "twenty X's" or whatever.

So:
Two trucks came off the assembly line.
Twice as many trucks came off the assembly line.
Three times the previous number of trucks came off the assembly line.
Etc.

the meaning of the sentence is that the number of trucks in 1929 was almost twice 469,000.
choice (b) conveys this idea accurately, with the phrase "twice that number". see, "that number" must refer to an actual number cited in the sentence, and there is only one such number. mission accomplished.

the number指代不清

#2
"trucks themselves" doesn't make sense.
you don't use "x itself" unless you are trying to emphasize some element of the inherent nature of x (as opposed to something associated with x, or with some part of x).
example: pet accessories are becoming more and more popular, even though pets themselves have maintained constant popularity.

#1choice (b) conveys this idea accurately, with the phrase "twice that number". see, "that number" must refer to an actual number cited in the sentence
choice (d) DOES NOT convey this idea. that choice says "twice AS MANY trucks", which means "twice as many trucks as were produced in 1914".
the problem is that the sentence doesn't tell us how many trucks were produced in 1914

为什么number这个noun可以发出came off这个动作？

this truly could be a tricky one...

numbers。。。给跪了，错选A

D&E.the last two choices, however, are problematic, because "twice as many trucks" must mean exactly that--i.e., twice as many trucks. as in, 2 乘以(previous # of trucks).
this sentence contains no information about the previous number of trucks; there's only a combined figure for trucks and cars. so, that doesn't work.

Twice做限定语，后面直接加数量类的名词 Twice that number
不能选DE， 是因为从前文推不出来Truck有多少量，所以何来Twice as many

D选项错在哪里？不可以用many吗？

twice后面要跟单数的number

that number 指的是之前出现过的那个数字