Well Deidara's explosion can cover 10km radius. That's 314 square km.
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Nagato's CT looks bigger. You can tell by looking at the curvature of the land under the CT sphere.
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Now a human city like Tokyo, for example, has 13 million people in 2188 squared km. Under the right conditions, there can be an equally dense area in the Narutoverse. For example, if ninjas moblize for war, then gather in tight spaces for formation. Imagine a whole fleet like this. Well, let's assume that its possible in the narutoverse to have a city as dense as tokyo.
If we assume that Nagato's CT can cover a radius 10km, like Deidara, then according to may calculations, CT can take out 1865630.7 , thats almost 2 million.
But the number should higher than that since CT appears to be larger than C0, meaning its radius of effect should be more than 10km; and it should actually be that way since Nagato's strength was feared even within Aktsuki itself.
Now you could make the argument that there isn't that many people in the narutoverse itself. But my point is still proven, Nagato can, under the right conditions, take out over a million ninjas.