The guiding principle is that if a number is a potentially dynamic one changing very often, then numbers will be the one. For example, if we are reporting on the ever-changing index figure of Nasdaq on a particular day, we cannot perhaps denote it with a particular number or at a particular number.
On the contrary, if we want to report at 11.00 am the number of students who took the GMAT on a given date at a particular center, one hour after entry is closed, then that figure could be reported as 'the number' as that number is not going to change. So the usage is more contextual rather than conceptual.
But, The given topic doesn't entail a discussion on this issue as all the five choices have the word 'numbers'. However, to generalize that the number or a number is the only acceptable form is rather to overreach. If that were so, GMAT would not have used it elsewhere as given below.
1. The gyrfalcon, an Arctic bird of prey, has survived a close brush with extinction; its numbers are now five times greater than when the use of DDT was sharply restricted in the early 1970’s. (
OG verbal review)
(A) extinction; its numbers are now five times greater than
(B) extinction; its numbers are now five times more than
(C) extinction, their numbers now fivefold what they were
(D) extinction, now with fivefold the numbers they had
(E) extinction, now with numbers five times greater than
Ans A.
2. With an awareness that there are connotations associated with the numbers 1 and 2 and the letters A and B, companies conducting consumer taste tests of foods or beverages typically choose numbers such as 697 or 483 to label the products. (GMAT Prep)
(A) With an awareness that there are connotations associated with the numbers 1 and 2 and the letters A and B
(B) Because the numbers 1 and 2 and the letters A and B have connotations they are aware of
(C) Because of an awareness of the numbers 1 and 2 and the letters A and B having connotations
(D) Aware of the connotations of the numbers 1 and 2 and the letters A and B
(E) Since the numbers one, two, and the letters A and B have connotations associated with them and they are aware of it
Ans: D