How to count the inserted documents on changeStream of mongo watch method?

Hello! I use mongodb changeStream with watch method and my mission is to pack 100 documents from changeStream. Below is my basic sources.

mongo = MongoClient('localhost', 27017, replicaSet='replica01')

target_coll = [
        'coll_earn_Construction_month', 'coll_earn_Education_and_Health_Services_mon',
        'coll_earn_Financial_Activities_month', 'coll_earn_Goods_Producing_month'    

database = 'etlmongodb'

pipeline = [{'$match': {'operationType': 'insert'}}, {'$match': {'ns.coll': {'$in' : target_coll}}}]
results = mongo[database].watch(pipeline)
from pprint import pprint
for result in results:

These codes work correctly. But I have no idea how to count the returned documents number. I want to know how to recognize the number of returned documents reaches 100 with pymongo api.
Any idea will be deeply appreciated. Best regards