How secure is your password on iCloud.com?

I'm wondering how secure a password is on iCloud.com. I thought after so many unsuccessful attempts to guess a password, I would be locked out. However, I tried entering wrong passwords about 20 times to my iCloud.com account, and I was never locked out. I was then able to enter my correct password and log in. I do have two factor verification enabled, but it is not required for iCloud.com.

How many attempts will force a lockout? I am wondering if someone were to be able to break in using brute force.