Once numbering in the millions worldwide

IMO E
Because in all the other options the comparison is nonsensical

To me this question highlights the value of finding a tight decision point. While on the GMAT using the word with tends to be incorrect if something is not actually with something else, I am not really sure whether some is preferable to with approximately. So I am going to look for other decision points.

(A) This conveys that the wolf has declined to a number. The number of wolves could decline to a number, or the population of wolves could decline to a number, but saying that the wolf itself has declined to a number does not really make sense.

(B) This one nonsensically conveys that the wolf itself has been estimated.

(C) In this version the wolf has declined to an estimate. That doesn't make sense either.

(D) Now multiple wolves have declined to an estimate. This one is not much different from C.

(E) Hmm. Now multiple wolves have declined to a number. I guess this one is better than A, in that rather than saying that THE wolf has declined to 200,000, it can be seen as saying that wolves have declined to 200,000 wolves. So I guess that aspect of this version basically works. Meanwhile, this version conveys that the number, rather than the wolves, is what is estimated, and that makes sense.

So, the best answer is E.