Generate Unique Number in Python within the max limit of Postgres bigint datatype

i have a problem statement where i need to find the unique number in python and save it in database to PostgreSQL .Every time it should generate unique number (i.e should not be repeated)

Also i need to append one prefix as well to the unique number :

The solution I Tried till now :

import time

def generate_unique_reference_number(order_type_id=28):
        order_type_prefix = '{:<02}'.format(order_type_id)  # 2 digit
        current_time = str(int(time.time() * 1000000))  # 16 digit
        return int(order_type_prefix + current_time)

numbers = []
numbers_set = set()
for i in range(10000):
    num = generate_unique_reference_number(28)
    numbers.append(num)
    numbers_set.add(num)

print(len(numbers))
print(len(numbers_set))
print(len(numbers) - len(numbers_set))

Its still giving me duplicate values .

Max limit for bigint in postgres server is :

-9223372036854775808 to 9223372036854775807

i can use nano seconds(i.e 19 digits limit same as postgres) ,but i need to append prefix of 2 digits as well .

def generate_unique_reference_number(order_type_id=28):
            current_time = str(int(time.time_ns()))  # 19 digit
            return int(current_time)

even nano seconds approach return duplicates but lesser count .