Why nodejs is taking CPU?

I am new to node. I was writing a sample http server is node http module. After profiling it with jmeter

Jmeter Test Case: Number of threads: 20 Delay after every request : 5ms

Node cpu usage is varying 17%-20% with a simple program.

Node version: v0.10.0 Sample code:

var http = require('http');
var url = require("url");
http.createServer(function (req, res) {
    var uri = url.parse(req.url).pathname;


    var body = "";

    req.on('data', function (chunk) {
        body += chunk;
    });

    req.on('end', function () {     
        res.writeHead(200, {'Content-Type': 'text/plain'});            
                res.end('hi vivek');
    });


}).listen(9097, "127.0.0.1");

In my sample program I am not doing any cpu task. My questions are?

1. Why it is taking that much cpu?
2. How many parallel user request can I support in node?