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