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I'd say "Иван здесь? -- не, нету" или "не, его нет".

I pretty rarely hear "Иван здесь? -- нет, Иван не здесь", it sounds mockingly a bit.

By the way, it's an interesting phenomenon, why singular "человек" is used to refer to multitude ? "человек" has no plural form at all.

Some people say живёт, some живут. I don't see any difference, maybe живут is more correct from the grammatical point of view. But both are good. "живёт 20 тысяч человек" most likely is understood as...