You don't need to get all user followers in order to count them. Use followers_count
property. E.g.:
import tweepy
auth = tweepy.OAuthHandler(..., ...)
auth.set_access_token(..., ...)
api = tweepy.API(auth)
for user in tweepy.Cursor(api.followers, screen_name="twitter").items():
print user.screen_name, user.followers_count
Prints:
...
pizzerialoso 0
Mario98Y 0
sumankumarjana 1
realattorneylaw 3056
JaluSeptyan 10
andhita_khanza 18
...
Hope that helps.