How to run a function multiple times and return different result python

I'm having a problem and I don't know if this is doable but what I wanted to do is call a function multiple times. The idea is I am generating a serial number for tickets, I have a function that generates a number which is 16 in length:

import random
import string

def random_number_generator(size=16, numbers=string.digits):
    return ''.join(random.choice(numbers) for _ in range(size))

Now if I run this function like this print(random_number_generator() * 4) it will return something like this:

1234567890987654
1234567890987654
1234567890987654
1234567890987654

How do I make it to return different numbers instead of the same numbers?