How many DC need In my Scenario

hello to dears

I have Scenario that have one main office that has 400 Users.

In scenario i said 1 forest and 1 doamin with 2 DC's

one of them for fail tolerance .

but he doesnt accept my reason

any body give me alot of reason for having 2 Dc ?

I appreciate from answer.

YaHyA.ZaHeDi