Steemit node:
s = Steem(nodes=["https://api.steemit.com"])
print(s.steemd.get_account_count())
3619629
Other nodes:
s = Steem(nodes=["https://api.moecki.online"])
print(s.steemd.get_account_count())
1894124
See the post here: https://steemit.com/steem/@justyy/qqvkj-how-many-accounts-on-steem-blockchain