Run a for loop parallelly in batches in Python

I would rearrange the code as follows:

1. Create "worker" function process_user that will process a single user. if your pool size is 4 then 4 of users can be proccessed concurrently each by a process in the multiprocessing pool.
2. process_user will now access what were previously arguments, i.e. instance_id, region_name, env ands3_bucket, as global variables, which have been defined in each pool process using the initializer and initargs arguments of the multiprocessing.pool.Pool.__init__ method. Likewise, a single global client instance that can be reused by process_user will be created for each pool process.

This is the general idea, which obviously I cannot actually test. You need to study this and understand what is being done with the help of the Python docs so that you can make whatever changes are required to get this to run.

import json
import boto3
from pprint import pprint
import os
import botocore
from datetime import datetime
import csv
from botocore.client import Config
import logging
import urllib3
import sys
import time
from boto3.session import Session
from botocore.client import Config
import multiprocessing as mp

LOGGER=logging.getLogger()
LOGGER.setLevel(logging.INFO)

MaxResults = 4

config = Config(connect_timeout=7200, 
                retries={'max_attempts': 5},
                proxies={'http': os.environ.get('HTTP_PROXY'),'https': os.environ.get('HTTPS_PROXY'),}
)

def init_pool_processes(*args):
    global instance_id, region_name, env, s3_bucket, client
    # Unpack:
    instance_id, region_name, env, s3_bucket = args
    
    urllib3.disable_warnings(urllib3.exceptions.InsecureRequestWarning)
    
    # One global client per process:
    client = boto3.client('connect',region_name="us-east-1",verify=False, config=config)  

def process_user(user):
    #time.sleep(0.1)
    user_id=user['Id']
    user_name=user['Username']
    response=client.describe_user(
        InstanceId=instance_id,
        UserId=user_id
    )
    user_firstname=response['User']['IdentityInfo']['FirstName']
    user_lastname=response['User']['IdentityInfo']['LastName']
    user_name=response['User']['Username']
    user_routing_profile_id=response['User']['RoutingProfileId']
    user_security_profile_ids=response['User']['SecurityProfileIds']
    #user_email=response['User']['IdentityInfo']['Email']
    user_phonetype=response['User']['PhoneConfig']['PhoneType']
    user_desk_number=response['User']['PhoneConfig']['DeskPhoneNumber']
    user_autoaccept=response['User']['PhoneConfig']['AutoAccept']
    user_acw_timeout=response['User']['PhoneConfig']['AfterContactWorkTimeLimit']
    try:
        user_hierarchy_group_id=response['User']['HierarchyGroupId']
    except KeyError as e:
        user_hierarchy_group_id=""
      
    security_profile_name_list=''
    
    hierarchy_group_name=""
        
    user_info=""+user_firstname+","+user_lastname+","+user_name+","+routing_profile_name+","+security_profile_name_list[1:]+","+user_phonetype+","+user_desk_number+","+str(user_autoaccept)+","+str(user_acw_timeout)+","+hierarchy_group_name+"\r\n"

    print(user_info) 

def getUserList(instance_id: str, region_name: str, env: str):
    
    users_all = ''
    client = boto3.client('connect',region_name=region_name, verify=False, config=config)    
    
    
    #get list of Amazon Connect users - first batch
    response = client.list_users(
        InstanceId=instance_id,
        MaxResults=MaxResults
    )
    users=response['UserSummaryList']
    
    while "NextToken" in response:
        response = client.list_users(InstanceId=instance_id, MaxResults=MaxResults, NextToken=response["NextToken"])
        users.extend(response['UserSummaryList'])
    
    users_all = [*users_all,*users]
    
    return users_all

def lambda_handler(instance_id: str, region_name: str, env: str, s3_bucket):
    #get list of Amazon Connect users
    users=getUserList(instance_id,region_name,env)
    
    pool = mp.Pool(4, initializer=init_pool_processes, initargs=(instance_id, region_name, env, s3_bucket))
    pool.map(process_user, users)
    pool.close()
    pool.join()

if __name__ == '__main__':
    instance_id = os.environ.get('instance_id')
    region_name = os.environ.get('Region')
    s3_bucket = os.environ.get('s3_bucket')
    env = os.environ.get('export_environment')

    lambda_handler(instance_id,region_name,env,s3_bucket)

You also need to look at:

    users_all = ''
    ... # code elided
    users = response['UserSummaryList']
    ... # code elided
    users_all = [*users_all, *users]
    return users_all

Isn't the above equivalent to the following?

    users = response['UserSummaryList']
    ... # code elided
    return users