もちろん、複数のcsvファイルを一度に処理できるようにコードを変更することはできます。
以下が変更後のコードです：
```
import csv
import requests
import tkinter as tk
from tkinter import filedialog
from tkinter import messagebox
from tkinter import simpledialog

def process_csv(csv_file, output_path):
    with open(csv_file, "r") as file:
        reader = csv.reader(file)
        header = next(reader) # skip the header row
        for row in reader:
            name, url, rating = row
            response = requests.get(url)
            if response.status_code == 200:
                subfolder = output_path + "/" + rating
                if not os.path.exists(subfolder):
                    os.makedirs(subfolder)
                file_path = subfolder + "/" + name + ".png"
                with open(file_path, "wb") as f:
                    f.write(response.content)
            else:
                messagebox.showerror("Error", f"Failed to download {name} from {url}")

def select_csv():
    csv_file = filedialog.askopenfilename(initialdir = "/", title = "Select CSV file", filetypes = (("CSV files", "*.csv"), ("all files", "*.*")))
    if csv_file:
        return csv_file

def select_output_path():
    output_path = filedialog.askdirectory(initialdir = "/", title = "Select output path")
    if output_path:
        return output_path

root = tk.Tk()
root.withdraw()
csv_file_number = simpledialog.askinteger("Number of files to open", "How many files do you want to open?")
for i in range(csv_file_number):
    csv_file = select_csv()
    if csv_file:
        output_path = select_output_path()
        if output_path:
            process_csv(csv_file, output_path)
            messagebox.showinfo("Success", "Processing completed")
```
新しいプログラムでは、tkinterライブラリのsimpledialogモジュールを使ってユーザーに開きたいファイルの数を尋ね、その数だけプログラム本体を実行します。
Sure, I can change the code so that it can process multiple csv files at once.
Here is the altered code:
```
import csv
import requests
import tkinter as tk
from tkinter import filedialog
from tkinter import messagebox
from tkinter import simpledialog

def process_csv(csv_file, output_path):
    with open(csv_file, "r") as file:
        reader = csv.reader(file)
        header = next(reader) # skip the header row
        for row in reader:
            name, url, rating = row
            response = requests.get(url)
            if response.status_code == 200:
                subfolder = output_path + "/" + rating
                if not os.path.exists(subfolder):
                    os.makedirs(subfolder)
                file_path = subfolder + "/" + name + ".png"
                with open(file_path, "wb") as f:
                    f.write(response.content)
            else:
                messagebox.showerror("Error", f"Failed to download {name} from {url}")

def select_csv():
    csv_file = filedialog.askopenfilename(initialdir = "/", title = "Select CSV file", filetypes = (("CSV files", "*.csv"), ("all files", "*.*")))
    if csv_file:
        return csv_file

def select_output_path():
    output_path = filedialog.askdirectory(initialdir = "/", title = "Select output path")
    if output_path:
        return output_path

root = tk.Tk()
root.withdraw()
csv_file_number = simpledialog.askinteger("Number of files to open", "How many files do you want to open?")
for i in range(csv_file_number):
    csv_file = select_csv()
    if csv_file:
        output_path = select_output_path()
        if output_path:
            process_csv(csv_file, output_path)
            messagebox.showinfo("Success", "Processing completed")
```
The new program uses the tkinter library's simpledialog module to ask the user how many files they wish to open, and then it runs the main body of the program that amount of times.