From what I heard, you can sometimes refer to the same word as plural or singular. It depends on what's important for you.
The community are glad have a lot of money. (a community made out of many people)
The community is glad to have a lot of money. (only a community)
The communities are glad to have a lot of money. (many communities)
At the same time the word "people" is singular and plural anyway ("many people" is not "peoples"). Confusing.
Back to your example: an estimated [2 million people] are
a [group of 2 million people] are
("estimated 2 million people" is to be seen as one element)
You could also say: a [group of people] is
Now use what I just told you about treating a group of people as plural; and you get the following:
an [estimated 2 million people] are
1
For the use of a/an in a phrase regarded as plural, see some examples in this answer. In fact, the question of "An estimated 2 million people" falls into similar patterns also shown in that my answer.
– None – 2014-12-23T14:14:43.7301Insert "an estimated (number of) 2 million people" and you will understand this elliptic structure better. – rogermue – 2014-12-23T17:53:11.147
2Your example involves a parsing where the article "a" is not the determiner for the matrix NP, e.g. "[a good] three hefty steaks" (CGEL pg 353, [70.iv]). Also, consider "[an additional] two million people". – F.E. – 2014-12-23T18:12:25.837
I have to disagree with @rogermue, I'm afraid. That phrasing confuses the issue even more because it makes "number" the subject instead of "people." – Jesse – 2014-12-23T19:00:00.803
@Jesse But "a number of X" is normally used with plural verbs (though singular verbs is also possible). – Damkerng T. – 2014-12-23T19:09:28.590
1@DamkerngT. now that I think of it, you're right. "A number of people were asked…", "A number of meals were served…". I still think it fails to clarify the question, though, since it leaves the same "why is this verb plural/singular?" question as before. – Jesse – 2014-12-23T19:17:41.153
@Jesse Longman DCE: Doctors believe only a tiny number of people are at risk. The verb form after "people" can grammatically refer to "number" (is at risk) or more logically to "people" (are at risk). And I think the latter is preferred. – rogermue – 2014-12-23T22:00:41.963