A simple script that will download information about sets and determine the user who owns the most sets.

User_set_info.js

const fs = require('fs');
const request_main = require('request');

function request(url) {
	let options = {url:url, headers: { 'User-Agent': 'Downloading user set information' }};
	console.log(options);
	return new Promise(function (resolve, reject) {
		request_main(options, function (error, res, body) {
			console.log(res.statusCode);
			if (!error && res.statusCode == 200) {
				resolve(body);
			} else {
				reject(error);
			}
		});
	});
}
async function start(){
	for(let i = 1; i < 41; i++){
		try{
			const text = await request('https://e621.net/set/index.xml?page='+i);
			await writefile('/path/to/file'+i+'.xml', text);
			console.log('wrote '+i);
		} catch(e) {
			console.log(e)
		}		
	}
}

start();

async function writefile(path, tex){
	return new Promise(function(resolve, reject){
		fs.writeFile(path, tex, function(err){
			resolve();
		})
	});
}

// grep -hEi "<user_id type=\"integer\">([0-9]*)</user_id>" *.xml | grep -oEi '[0-9]+' | sort | uniq -c | sort