DownloadFileCompleted being executed before the file has finished downloading

DownloadFileCompleted is running before the file has finished downloading resulting in files with 0 bytes and a wrong counter.

Files with 0 bytes

foreach (var img in image_list)
{
	await Task.Run(() => DownloadFile(img));
}

private async Task DownloadFile(JToken file) {
	using (WebClient client = new WebClient()) {
		client.DownloadFileCompleted += DownloadFileCompleted(file["name"]);

		client.DownloadFileAsync(new Uri(file["url"]);
	}
}

private AsyncCompletedEventHandler DownloadFileCompleted(string nome) {
	total_download++;
	label1.Text = total_download;
}

I would like to download several files in parallel and as the file finishes downloading, a count is incremented but the count is being incremented before the download is completed (images with 0 bytes are being counted as a complete download).

I don't recommend multithreading approach for IO operations, since thread waits (more precisely speaking it is unnecessarily hold in app thread pool - multithreading of IO operations results in unoptimal resource usage in the end) for IO operation to complete, doing nothing in meantime.
If you want to make your current code work though, it can be fixed by adding async and await:

foreach (var img in image_list)
{
    await Task.Run(async () => await DownloadFile(img));
}

private async Task DownloadFile(JToken file) {
    using (WebClient client = new WebClient()) {
        client.DownloadFileCompleted += DownloadFileCompleted(file["name"]);
        await client.DownloadFileTaskAsync(new Uri(url), "some save location");
    }
}

private AsyncCompletedEventHandler DownloadFileCompleted(string nome) {
    total_download++;
    label1.Text = total_download;
}

If you want to create the most optimal solution, i recommend to get to know about IAsyncEnumerable, which gives you possibility of aynschronous iterating over collection.

You should just use HttpClient instead of WebClient, and use async and await everywhere, rather than just hacking Task.Run to offload the download.

static HttpClient _Client = new();

foreach (var img in image_list)
{
    await DownloadFile(img);
    total_download++;
    label1.Text = total_download;
}

private async Task DownloadFile(JToken file)
{
    using var stream = await _client.GetStreamAsync(file["url"]);
    using var fs = File.OpenWrite(file["name"]));  // or other path here
    await stream.CopyToAsync(fs);
}