[python] Send a file through sockets in Python

I'm trying to make a program in python that implements sockets. Each client sends a PDF file and the server receives it and the title is changed to "file_(number).pdf" (e.g.: file_1.pdf). The problem presented is that only a client can send a file successfully. When a second client tries to send the file, the program crashes. What am I doing wrong and how can I solve my code to allow N clients (with N < 20) to connect to the server and transfer files?

Here's the server code:

import socket
import sys
s = socket.socket()
s.bind(("localhost",9999))
s.listen(10) # Accepts up to 10 incoming connections..
sc, address = s.accept()

print address
i=1
f = open('file_'+ str(i)+".pdf",'wb') # Open in binary
i=i+1
while (True):

    # We receive and write to the file.
    l = sc.recv(1024)
    while (l):
        f.write(l)
        l = sc.recv(1024)
f.close()

sc.close()
s.close()

Here's the client code:

import socket
import sys

s = socket.socket()
s.connect(("localhost",9999))
f = open ("libroR.pdf", "rb")
l = f.read(1024)
while (l):
    s.send(l)
    l = f.read(1024)
s.close()

To simplify my code, I always use a book with file name "libroR.pdf", but in the full code it is chosen by a GUI.